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Paint blister proposal for Vans resolution

As I mentioned earlier, since I am getting ready to apint soon, and I have had the blisters on 2 of my past QB's, I am really anxious to not have this happen again. So, I'be been "'sperimenting" over the last few weeks with some different solvents, and I have noticed a few things.
First, I always have final cleaned with acetone prior to acid etch primer, so I did the same with my 2 RV-10 QB tanks now. After a couple of days, I noticed some weeping contamination around 60%-80% of the rivets. I can't tell what it is, other than to say it looks like a slight film contamination, almost like a water stain, and it does grow as time passes to around 2 diameters of the rivet head, and is not always perfectly round.
So now, I have also celaned them with Dupont Prepsol (which is really mineral spirits), and MEK. There is almost no contamination evident with these 2 cleaners after a couple of days, but I will wait until the week end to declare victory.
On the RV-7 which is beginning to look absolutely terrible, there aren't any fuel stains underneath the blisters. When they do pop, it looks like an adhesion issue with the paint/primer. Carol and I do actually remember this same kind of phenomenon (weeping film) when we painted them, but really didn't think much of it at the time. We just rewiped, and then shot the primer.
I noticed that our QB parts have some kind of oil on them when received from VAN's. I've assumed it is some kind of protectant for the overseas journey. I noticed that I saw this same kind of weeping on the screw holes on the tank where I had not cleaned them well enough, and it got me to thinking that perhaps the oil is reacting or absorbing into the proseal and weeping out over time, causing the adhesion issue. Since it doesn't occur anywhere else on the airplane, I'm thinking it could be related to the proseal, but for different reasons than have been suggested, such as an improper mix, especially since there doesn't appear to be fuel underneath the affected rivets. Of course, perhaps an improperly mixed proseal works good enough to be leak free, but can still aborb and weep contaminants on the surface.

Vic

Vic
 
I Think Vic Is On To Something...

Anecdotally, I think Vic is on on to something with his premise that it may have something to do with the oil preservative used on QB tanks interracting with the Proseal, and if not addressed properly prior to painting could cause the blistering problem. If so, focussing on testing in this area may bear fruit. If the majority of problems exist with QB tanks, and the major difference between QB tanks and standard-build tanks is the use of preservative oil, then this would make sense to me.
 
different opinion

I still get paint new blisters on my tanks, long after solvents in the paint have evaporated (9+ years since paint, 1100 hours, and lots of time with the airplane sitting out in the sun.) One thing I think folks are missing here is that it takes pressure to form a bubble in the paint. I've had paint blisters as large as a dime. They occur both top and bottom, but only about midpoint on the bottom out; curiously in areas that are above typical fuel levels in the tanks. My theory is the vent lines always have fuel trapped in them which causes a pressure buildup in the tank as vapor expands. It takes a bit of pressure to for trapped fuel to flow out the vent lines going uphill 18" or so at the loop up then back down in the fuselage. I've never seen a vent line dribble; usually just comes out at once like an upside-down miniature version of Old Faithful. As the airplane sits with pressurized tanks, over time the vapor works through any pores or voids in the proseal, thus causing the paint to bubble. If one studies how vent lines are implemented on other airplanes, you will find that they always will drain downhill so the vent lines stay clear of fuel. Cessnas, for example, typically employ a check valve with a small drain hole to allow for drainage. This isn't the case with the vent lines as designed on brand-V airplanes, fuel will stay trapped in the lines.

If one theorizes proseal or solvent outgassing causes blistering, a good test would be to cover a scrap piece of proseal, wait a few hours and paint it. Then throw it in the oven for a few hours at 150 degrees.
 
rocketbob said:
One thing I think folks are missing here is that it takes pressure to form a bubble in the paint. .... It takes a bit of pressure to for trapped fuel to flow out the vent lines going uphill 18" or so at the loop up then back down in the fuselage.
Click to expand...

About 0.48 psi.

Can anyone with vent lines coiled in the wing root report a tank blister?
 
vic syracuse said:
.....I have had the blisters on 2 of my past QB's.
Click to expand...
Sparky said:
Anecdotally, I think Vic is on on to something with his premise that it may have something to do with the oil preservative used on QB tanks....
Click to expand...
Exactly how does the preservative theory square with standard kit builders who sometimes report the identical condition, even though the standard kit contains no preservative?

It can be difficult for any person who has not actually constructed fuel tanks to understand exactly what is required applying proseal to create a leak proof "pressure boundary."

This issue has been discussed for years and many theories have been offered, some more creative than others. The only undisputed piece of evidence that links a SB and QB assembled fuel tank is one simple fact: Fuel tanks are assembled using hand labor. I remain convinced the overwhelming number of paint blisters that occur on fuel tanks comes down to the quality of that hand labor. When it comes to fuel tank construction, certain fundamentals need to be understood AND adhered to by applying specific techniques unique to fuel tank construction. There is no substitute for attention to detail. Those details are very important and most certainly DO NOT include thinning proseal with MEK as more than a few builders have reported. That is only one detail. Regardless of who builds the fuel tanks, other details include assuring the integrity of the fay/fillet seal. If that seal is defective, most commonly because it contains hidden voids, leaks not only can occur but will occur. Leaks will occur because a fuel or vapor will do what the law of physics demand: find the path of least resistance. It will accomplish this by taking advantage of a leak path. For blisters to occur over a rivet head, the leak path is the next possible exit for fuel or vapor....through its rivet hole.

Properly sealed, fuel tanks can take a lot of pressure...way more than the location of a vent line can ever generate.

63sifs.jpg


Finally, I don't care how many times you strip the paint and start over using the finest prep work humanly possible. If those interior issues are not properly dealt with, the chances are good that sooner or later new paint will blister again. It is that simple.

To my mind, the only practical repair short of building new fuel tanks is to use something that has been outlawed for a number of years...slosh. Even then, such a "fix" may or may not work. Worse, the use of slosh may introduce a whole new set of problems that go well beyond dealing with a mere "cosmetic" issue and enter into the realm of flight safety.
 
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DanH said:
About 0.48 psi.

Can anyone with vent lines coiled in the wing root report a tank blister?
Click to expand...
I assume by coil, you mean run in such a way as to have some point of the vent higher than the tank, or a coil to introduce locations for fuel/water to sit in low points of the coil to build pressure.
8's, and most other RV's by design, have the vents come into the fuse, route up the gear tower and then down.
I wondered about vent pressures early on in my concerns in this thread
I do think Bob J is correct in that part of the dynamic is pressure build up in the tank.
 
Kahuna said:
I assume by coil, you mean run in such a way as to have some point of the vent higher than the tank, or a coil to introduce locations for fuel/water to sit in low points of the coil to build pressure.
8's, and most other RV's by design, have the vents come into the fuse, route up the gear tower and then down.
Click to expand...

I was thinking about the Rocket-style tank vents, a coil of vent tube inboard of the tank which doesn't enter the fuselage. The coil-style vent wouldn't have the more-or-less standard 18" rise Bob mentioned. I calculated 0.48 psi for an 18" fuel rise (inches of water x specific gravity of fuel converted to psi). A coiled vent line would be less.

Your previous thread reference was interesting, and as noted, dynamic vent pressure is higher than pressure generated prior to burping a tank vent on the ground.
 
Gentlemen, time to get hard evidence. This is not the Dark Ages.

First step is to confirm or eliminate fuel as a contributor to the blister problem. If we confirm fuel components in a blister, we know we have a liquid or vapor leak from inside the tank. If there is no evidence of fuel, we concentrate on factors external to the tank.

I've made arrangements for formal tests with a lab. They will use chemistry and a gas chromatograph to look for certain hydrocarbons as well as tetraethyl lead, a sure indicator of avgas. The testing is not expensive, less than $500 for maximum effort with a full report, and as little a $45 for basic checks which may answer the question immediately. The maximum effort will tell us a lot more than just fuel or no fuel.

We need one or more volunteers. The lab has requested about three drops of the mystery liquid found in some blisters. They instruct the best way to gather the samples is to purchase a new syringe, a #18 needle or smaller, and a sterile test tube, all of which should be available at a pharmacy.

It may also be useful to slice a few whole blisters off an airplane and supply those with a paint sample from the same aircraft. A sample of cured Flamemaster CS-3204 would also be good. Details to follow; I need to send the lab a basic information package so they can suggest specifics.

Financial contribution would be nice. I'd guess there are at least 500 readers here with QB fuel tanks not yet painted, all concerned with avoiding a future blister problem. Then there are the guys currently stuck with a blister problem and wishing they could repaint with confidence. Given the price of paint and painters, a few bucks in the pot might be a good investment.

So who wants to nail this thing?
 
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DanH said:
I've made arrangements for formal tests with a lab. They will use chemistry and a gas chromatograph to look for certain hydrocarbons as well as tetraethyl lead, a sure indicator of avgas. The testing is not expensive, less than $500 for maximum effort with a full report, and as little a $45 for basic checks which may answer the question immediately. The maximum effort will tell us a lot more than just fuel or no fuel.

Financial contribution would be nice. I'd guess there are at least 500 readers here with QB fuel tanks not yet painted, all concerned with avoiding a future blister problem.

So who wants to nail this thing?
Click to expand...

OK, I admit I am one of the folks waiting/watching to find an answer before painting the plane.

Post, or PM your address, and I will kick in.
 
DanH said:
I was thinking about the Rocket-style tank vents, a coil of vent tube inboard of the tank which doesn't enter the fuselage. The coil-style vent wouldn't have the more-or-less standard 18" rise Bob mentioned. I calculated 0.48 psi for an 18" fuel rise (inches of water x specific gravity of fuel converted to psi). A coiled vent line would be less.

Your previous thread reference was interesting, and as noted, dynamic vent pressure is higher than pressure generated prior to burping a tank vent on the ground.
Click to expand...

The loop is really not an improvement in this regard, and depending on how many loops there are, can make the problem worse.

The easy solution is to have a flapper-type check valve, with a small hole drilled in it $ 0.040" or so, and just go straight down with the vent line from the vent fitting on the tank. Simple, easy, cheap.

One other thing I'll add to this discussion, there are many RV's flying now with prosealed trailing edges. I've never heard of paint bubbling up on the trailing edge.
 
Guys, it's not about the money. I'll front the money and worry about getting it back later. If ya'll don't send a friggin' dime it won't break me, and I'm not painting my tanks without an answer.

What we need are test samples from finished airplanes with blister problems. And understand there is no guarantee. The lab work may precisely define the problem, or it may not, and even knowing the problem doesn't necessarily produce a solution.

It will beat where we are now.
 
Money where my mouth is

I am interested in participating. My plane was abandoned at another airfield a couple of hours from here by my partner when the weather turned bad on Sunday. When it comes back, hopefully today, I will look at the possibility of cutting off one of the bubbles. I know one had blistered and popped on the bottom of the wing but it might be contaminated with fuel from spillage on filling the tank. So far none of the top ones have popped.

My only concern is that once I pop the blister then I might be opening myself up for a rapid delamination of the paint and it will force the issue of getting it repainted sooner than later. Not sure how many bullet hole stickers I can safely put on the plane :)

Dan H, I will pm you once I make the decision and get shipping instructions. I will try to videotape the process of removal so that it will show that I didn't contaminate the sample.
 
andrew phillips said:
Money where my mouth is....My only concern is that once I pop the blister then I might be opening myself up for a rapid delamination of the paint and it will force the issue of getting it repainted sooner than later.
Click to expand...
Excellent, Andrew! Thanks for stepping up. If my long held suspicion proves correct and fuel is causing the blisters, repainting will solve absolutely nothing and its a good bet sooner or later, new blisters will reappear anyway. I'm sure any number of doubters would love to see this theory disproven. Me too because if I'm right, there will be no easy fix. In any event, right or wrong or indifferent, we ALL want to see this issue nailed to the wall and at the very least....identify what is behind all these growing reports. The problem is not going to fix itself. Okay guys, who's next?
 
pressure test?

With slow build tanks I haven't followed this thread until now.

After reading every post I didn't hear anyone with a quick build tank say they pressure tested their tanks before installation, so here goes.

Did any of you with problems pressure test your QB tanks before installation?

Maybe that's why the SB tanks aren't having the same issue, or at a lower rate.
 
Yes, I did pressure test the RV-7 QB tanks with the balloon method. They held for over a week.

Vic
 
Oh those blisters

I have read all of the post on fuel tank blisters! I have also fallen victim to the sneaky little pest. Ok here it goes. My kit was delivered QB 2007. The wings were stored in the hanger until early 2011. Ok I'm slow. I did pressure check both tanks before paint. I have 3 colors going across the tanks. I have blisters on white and black areas. The paint is PPG base/clear. Blisters only on the top at this time. Most are the same size as the rivet head. I only use 100LL. I do not always keep my tanks full. My airplane is hangered and has only sat outside at Oshkosh last year and 1 week in Florida this April.
I guess my question is this. For everyone that had blisters in the 2009 time frame. Did you sand and repaint? If you did is it holding up ok? I have inspected both tanks via boroscope and the sealant is very thick over stiffeners and rivet heads. It's not what I would call a perfect job of sealing on the inside. But is ok. Just let me know what you think. Thanks for reading.
 
Ya'll are reading an old thread.

The oily protective coating is ordinary WD-40 sprayed at the QB facility in the PI.

We did collect data on a reasonable sample group, both QB and standard kits. Blisters are not just a QB problem, nor are they limited to novice builders. They've even shown up on some very pro-built USA assembled tanks. Standard kits without Pacific shipping have no WD-40 exposure, which casts doubt on the "weeping slime" theory.

Blisters from two different sets of tanks were sent to a lab. Lead residue was detected in one set and not the other, an inconclusive result.

We were never able to collect actual fluid from inside a blister; "wet" blisters seem to be rare. However, the few cases of reported fluid are unlikely to be anything other than fuel, as no other liquid is available at the blister site in the reported quantity.

Lacquer thinner, MEK, and drugstore 3% hydrogen peroxide do not cause sealant revision, i.e. the sealant does not melt like the Wicked Witch. That's not to say there is no physical effect, just no obvious effect.
 
Old thread but here's a (possibly) new idea. Find a set of blistered tanks that are being discarded (a wrecked plane, maybe). Then pressurize em real good to see if the blisters grow. Alternatively, remove the blisters and spray with suds.
 
Another set of photos

These photos are from my hangar-mate's RV-7A. Tanks are quickbuild from 2003-2004 era. The plane was painted in 2006 with Stewart System paint (apparently the water based paints are not immune from blisters). The plane has been stored for the last two years while the owner was getting his medical issues sorted out. All of the blisters were steadily growing (top and bottom of the tank) while sitting quietly in the hangar.

photo2.JPG


photo3.JPG


A few close up shots. Note the start of the small 'satellite' blisters surrounding the rivet. On the bottom of the tank, some of these outer blisters formed to be larger than directly over the rivets.

photo4.JPG


photo5.JPG


All of the blisters were dry (no fuel or other residue) when removed. The primer coat had small pockets or voids around the rivet. I'm not sure if this pitting has been seen on other peoples problem tanks. There was no evidence of the primer material sticking to the paint skin as it was peeled away.

photo6.JPG


photo7.JPG


Like many of you, we are reluctant to proceed with the re-paint until some root cause has been brought to light. I hope these photos contribute to the cause and stimulate some more thoughts.

-Mel
 
fuel for sure

Looking at these pictures, it is pretty hard to point to anything but fuel - fuel vapors as the cause of the blisters. Looking at the paint on the rest of the plane - I didn't see any anywhere else. Having painted my share of cars, boats, houses, and about everything else except a plane, I would not paint until I figured out how to seal the tanks better. It will take some work to smooth out around the pitted and blistered areas to hide them on the repaint.

I have changed my mind about having my wings QB. I think I will build them myself - at least then I have someone to blame!
 
Blisters

We had he same problem with an aluminium glider trailer that was sprayed with WD40 by the manufacturer 10 mins before we arrived to make it look shiney and nice when we went to pick it up. To say we were joyous about it was an overstatement.
We cleaned and cleaned the trailer before spraying it and did a good job. The primer had a very good bond, but between applying the primer and the top coat we came to the conclusion that some residual WD40 leached from the joints through the porous primer so when the top coat was applied it did not form a good chemical bondto the primer near the joints and we had the same result as can be seen on the pictures above where the primer looks as if it has a good bond with the tank but the top coat has a poor bond to the primer around the rivits.

As per usual these are only my opinions and maybe plain incorrect.

Rob
 
RivetDriver01 said:
The plane has been stored for the last two years -- All of the blisters were steadily growing (top and bottom of the tank) while sitting quietly in the hangar.

photo2.JPG


-Mel
Click to expand...

The fact there as many, and as large blisters on the top of the tank lead me to believe the vent line is not doing its job.

I suspect it takes more pressure to force fumes past the proseal, around the rivet, and under the paint than it takes to vent fumes through a functioning tank vent line.
 
My fix for the problem is simple - use no paint and polish the tank area. :)

Just kidding, but that inexpensive option is on the table with the 8. A nicely polished aluminum airplane always looks great.

Last summer I built new tanks for the 7 Jerry Cochran now owns and so far he has not reported blisters.....here's hoping it stays that way. For what it is worth, the new tanks have lots more sealant than the original units which were QBuilt back in 2002. Those tanks also appeared to have been assembled with a brush-able version of pro-seal, it was a very neat smooth job, something anyone would have been proud of but there was not a surplus of the stuff used.

This phenomena has got to be one of the great mysteries of our day. You'd think someone would be able to figure it out with all the technical knowledge we have these days. The original theory put forth to me by Vans was it was caused by surface contamination during the paint process - the resulting minute corrosion created a gas that lifted the paint. It occurs at the tanks because the rivets are sealed and the gas has no where to go except up lifting the paint. On other parts of the airplane it escapes down along the rivet shank and does not lift paint.

Interesting theory, it may be true. No one has disproved it. But neither has anyone come up exactly what is the cause of that contamination. My original tanks were painted with the rest of the airplane by a professional in a clean paint booth. The rebuilt tanks were painted by me with a Harbor Freight gun with no booth in open air. So go figure....same type of paint was used.
 
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rv8or said:
We had he same problem with an aluminium glider trailer that was sprayed with WD40 by the manufacturer 10 mins before we arrived to make it look shiney and nice when we went to pick it up. To say we were joyous about it was an overstatement.
We cleaned and cleaned the trailer before spraying it and did a good job. The primer had a very good bond, but between applying the primer and the top coat we came to the conclusion that some residual WD40 leached from the joints through the porous primer so when the top coat was applied it did not form a good chemical bondto the primer near the joints and we had the same result as can be seen on the pictures above where the primer looks as if it has a good bond with the tank but the top coat has a poor bond to the primer around the rivits.

As per usual these are only my opinions and maybe plain incorrect.

Rob
Click to expand...

Rob,

Did you get blistering like in the photos here? I never thought WD40 was a good idea, and some WD could be hiding in the rivet seams, but the blistering suggests to me there's some pressure behind it. Mike's suggestion of a vent problem is interesting, but sitting in a hangar it's hard to imagine there would be large swings in tank pressure - maybe not. My vote would be for fuel fumes since the blisters are dry. Others have suggested putting green locktite on the tank rivets. It's very thin and should work its way into any passages. Maybe I'll polish my 9....
 
RivetDriver01 said:
The primer coat had small pockets or voids around the rivet. I'm not sure if this pitting has been seen on other peoples problem tanks. There was no evidence of the primer material sticking to the paint skin as it was peeled away.

photo7.JPG
Click to expand...

The satellite blisters are interesting. The primer voids appear to be classic "solvent pop", tiny air bubbles trapped in the primer film. You can see the thin skin torn from the top of many the voids.

Most if not all theories for blister formation assume the primer to be porous. Quite a few photos suggest blisters commonly start at an underlying void. The exact transfer and blowing mechanism is not known yet, but it's not hard to imagine the transfer of a vapor pressure agent to a primer void at some small distance from a rivet.

BTW, was the primer choice EkoPoxy or EcoPrime?
 
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around we go....
I do not recall if anyone ever found any tank blisters like this on tanks built with proseal/flamemaster sealant before or after circa~2003 - 2007.
All of the other potential raised risk issues of paint, use of solvents, assembly technique, mixing ratios, etc, all happened before and after this period slow and fast build, yet we cant seem to find any blistered tanks before or after this period. Manufacturing sealant during this period the one common thread. I have hung my opinion hat on a manufacturing defect in sealant. We have evidence to suggest this is the case. No proof of anything but lots of data.
 
Last summer I built new tanks for the 7 Jerry Cochran now owns and so far he has not reported blisters...

Hi David,

Not a blister one...
 
I built my RV4 tanks with ProSeal from Van's. They were built side by side in identical jigs. Prep & paint were done simaltaneously with the same materials on the same day. My right tank developed blisters, on the top only, with no evidence of fuel leaks after 2 years. The left tank has no blisters and no leaks. They were built before 2003. I cannot think of any reasonable explanation for this, and it does not conform to any of the theories I have seen posted. The only difference in the two tanks is the inverted fuel flop tube in the right tank.
Comments???
 
Blisters

Tom

I think that the top coat doesn't bond properly to the primer coat of paint then another mechanism puts pressure, heat cycle? vapour from the proseal? the bubbles on the glidertrailer were around the rivits and there was no proseal but a mastic sealant. Because there is no proper bond there is a weak area that allows the paint to separate under the pressure.

As per usual That is my theory and it is probably wrong.

Cheers
Rob
 
Faulty sealant? Poor venting?

Hey Team, My 8 was not quick build, meets all the workmanship suggestions relating to tank rivet blistering, yet still developed tank rivet blisters. After removing tanks, stripping, treating, prime and repaint, the first day in a hot hangar with tanks full and bubbles reappear! I am convinced there is no smoking gun because there is no single root cause, but a complex combination including type of paint used, pressure in the tank, and perhaps faulty sealant.

Pressure: My tanks do not generate rivet bubbles when tank caps are left open! When full and caps closed, and hot, RV's will force fuel out the vent tube, this indicates that at least sufficient pressure is developing in the tanks to overcome the head of the fuel rising in the vent tube in the cockpit, before heading back down to the bottom of the fuselage. Many polymers, as far as I know, have some vapor transmission number, mean vapor transmission rate (MVTR), add pressure, and even a material with a very low MVTR and vapor transmission will increase.

Paint type: Van's are not the only type of aircraft that develop pressure, and other brands, and many Van's aircraft do not develop bubbles, so pressure alone is not the root cause. (The old Grumman line for one example develop leaks, but very few bubble issues.) My theory is that some paints have have higher MVTR's than others, I believe clear coats being the lowest. Add the low strength of some base coats, and the relatively low MVTR's of the clear coat, and you have an aggravating factor, at best. In addition to this, some paint chemistry may adhere better to the sealant that exists at the rivet fillet at the wing surface. For example, PPG automotive paint VS the PPG aircraft paint systems. The aircraft paint system being proven over sealant in the commercial and military aircraft market, the auto pant not. Some aircraft, like mine, have more sealant visible around the rivet heads than others. This would seam to mean better sealing, but is not the case, and may actually aggravate that situation through reduced paint adhesion in this area.

Bad Sealant: Can't rule out, based on data anyway, that some batches of sealant do not perhaps have a higher MVTR than others.

I'd like to know the paint brands and systems used on aircraft that both do and do not have paint blisters. Mine is the PPG Base Coat/Clear Coat system with catalyzed base coat.
 
Dave, Jerry, Did you have any issues with the new tanks lining up? The z-bracket holes etc? I am considering the same option.
 
mspenc45 said:
..... (The old Grumman line for one example develop leaks, but very few bubble issues.) ......
Click to expand...

Mark,

Not a good comparison... the Grumman tanks are mainly bonded and have very few rivets penetrating the wing skin.

The leaks they have are usually from the excessive thickness of proseal used inside the tank around the round wing spar and the end rib.
 
az_gila said:
Mark, Not a good comparison... the Grumman tanks are mainly bonded and have very few rivets penetrating the wing skin.
The leaks they have are usually from the excessive thickness of proseal used inside the tank around the round wing spar and the end rib.
Click to expand...

Excessive thickness?
 
DanH said:
Excessive thickness?
Click to expand...

Yes, as shipped new, some has been already removed -

tank015.jpg


With all of the factory stuff removed. Proseal just fills the gap under the spar - no real rivets or bonding here, mainly Proseal -

Day_5_Lower_Front_of_Spar%28r%29.JPG
 
az_gila said:
With all of the factory stuff removed. Proseal just fills the gap under the spar - no real rivets or bonding here, mainly Proseal
Click to expand...

Gil, I'm still not following. You're saying a thick layer of proseal is bad, or that this particular design lacks fasteners, and the designer expected proseal to bridge large gaps that flex and move?
 
DanH said:
Gil, I'm still not following. You're saying a thick layer of proseal is bad, or that this particular design lacks fasteners, and the designer expected proseal to bridge large gaps that flex and move?
Click to expand...

Both... :)

But really pointing out that the Grumman common leak spot has nothing to do with possible surface paint blisters since there are almost no rivets in the outer skin.

The common leak is at the top or bottom of the spar at the tank root rib, or around the fuel sender, which is almost identical to the RV setup.

The tank design dates back to around 1974.
 
I think you have to separate "leaks" from "blisters" in all cases. Survey evidence says wet blisters are rare. And thickness of a filet seal is unlikely to contribute to leak formation, unless the problem is lack of thickness.

FWIW, the lab was able to detect lead in a wet blister, confirming fuel rather than some other liquid source. They were not able to isolate lead in a dry blister sample. However, that fact is a bit inconclusive, as the vapor pressure of TEL is very low (0.2 mmHg). It would be difficult to transport much lead in vapor phase through sealant.
 
DanH said:
I think you have to separate "leaks" from "blisters" in all cases. Survey evidence says wet blisters are rare. And thickness of a filet seal is unlikely to contribute to leak formation, unless the problem is lack of thickness.

FWIW, the lab was able to detect lead in a wet blister, confirming fuel rather than some other liquid source. They were not able to isolate lead in a dry blister sample. However, that fact is a bit inconclusive, as the vapor pressure of TEL is very low (0.2 mmHg). It would be difficult to transport much lead in vapor phase through sealant.
Click to expand...

If in fact a pressure build in the tank is causing the blisters, relieving that pressure by loosening the fuel caps when parked would prevent it.

There have been reports of tank pressure build up due to some anomoly with the some vent systems. I do not understand how that could happened but it has been reported.

Perhaps the explanation is as simple as that. :)
 
Q: How many have used the Flamemaster, and how many have used 3M or DeSoto? Which goo does the factory use?
 
Any chances the issue is not the pro seal. . But the type of pro seal and the method of mixing.. when I was building the cj3 center wing box we would use premixed and frozen prc for the fuel tank fillets and laps.. it came from our supplier "spun" to remove the air out of the prc.. because as you mix it you beat a little bit of air into it just like making batter.. this air would be microscopic but it would be there. .


Maybe the secret is going to be how you mix your prc, which determines if you are going to get blisters. Because obviously in the homebuilt world you arent going to be able to buy the pre mixed and frozen pro seal.

If I remember correctly our pro seal also had micro balloons in it to add volume and reduce weight. . But don't quote me on that.

Mike
 
mmartin1872 said:
it came from our supplier "spun" to remove the air out of the prc.. because as you mix it you beat a little bit of air into it just like making batter.. this air would be microscopic but it would be there.
Click to expand...

Entirely true. However, the manufacturer's technical rep told me it's not possible to create a true solid, just smaller voids. Even the toluene solvent leaves voids when it evaporates...just turn up the magnification. Cured proseal is a closed cell sponge.

BTW, toluene evaporates out of the sealant for a few weeks, which tells us the stuff has a vapor transmission rate.

Taken at 10X:



aerhed said:
Which goo does the factory use?
Click to expand...

Flamemaster CS3204B2
 
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In applications where voids (like these) are not permitted common practice (where I worked) was to briefly subject the mixed batch to a very strong vacuum. The trapped air will 'boil off' over about 30 seconds. This was very effective at removing these voids but care needed to be taken if the products contained lower vp components. (The vacuum pot I used was purpose built with a 3/4" plexi lid.)
My experience with this process was with polyurethanes only, I never personally tested polysulphides, although I did give it some thought while building my tanks.
With the toluene reduction - I believe the addition of toluene is the only difference between the viscous and much less viscous product variations. Given this I once tested some samples with up to about 50% added to the Flamemaster mix and found a softer final product and significantly longer cure time. I did not test for changes to the properties beyond what I was specifically interested in at the time but I got the impression the toluene mixed at a molecular level so should not have introduced voids. Remember the outgassing is (or should be) diffusion at a molecular level.
 
Should I pop my blister

I have QB tanks and on top of one wing is a blister that has enlarged to the point it is bigger in diameter than the rivet under it. I wonder if it is time to pop the blister.

I personally think it has more to do with vapor. I live in a nice cool climate and this blister was hardly noticeable until I flew to OSH the first year I had it, it was warm there, the blister really bulged up. They plane has been flying for nearly 3 years now so the growth is very slow. I did not have any others till once when the plane sat with 1/3 tanks, now along the top and bottom at the rear spar there are very tiny blisters starting to form in a hand full of places.

Cheers
 
RVG8tor said:
I have QB tanks and on top of one wing is a blister that has enlarged to the point it is bigger in diameter than the rivet under it. I wonder if it is time to pop the blister.

I personally think it has more to do with vapor. I live in a nice cool climate and this blister was hardly noticeable until I flew to OSH the first year I had it, it was warm there, the blister really bulged up. They plane has been flying for nearly 3 years now so the growth is very slow. I did not have any others till once when the plane sat with 1/3 tanks, now along the top and bottom at the rear spar there are very tiny blisters starting to form in a hand full of places.

Cheers
Click to expand...

I've had the blisters on the QB tanks since it was painted 4 years ago. I used to get a lot, but now it's settling down. I keep an Exacto knife with a tiny #1 blade to lance the back side of the blister - it works most of the time, but in some cases, the blister eventually blows off anyway. In my opinion, the blisters are more frequent with warmer temps.

John
 
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