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Primer Question

3

330Jock

Well Known Member
OK I've read everything on the threads I can about priming. The question I have is if you use a self etching primer like Vans does, why would you want to scratch all that nice Alclad off before you sprayed the self etching primer on? Isn't the Alclad even more protection with the addition of primer? I have read the proponents of no prime & just leave the Alclad & prime only the non-Alclad surfaces. A little clarification please.
Bill
 
Scuffing the alclad with scotchbrite to prep for primer does not remove the alclad. It makes it dull looking, but its definitely still there.
 
The primer Vans uses is Sherwin Williams P60 G2 and the metal needs to be cleaned not scuffed. My understanding of the alclad is that it is only a few thousandth thick, so scuffing the metal would remove that protection. IMHO
 
My understanding of the alclad is that it is only a few thousandth thick, so scuffing the metal would remove that protection. IMHO
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That is extremely unlikely if you use scotchbrite, which is all you should use on alclad.
 
Joe Parish said:
......My understanding of the alclad is that it is only a few thousandth thick, so scuffing the metal would remove that protection. IMHO
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That is my understanding also, but I am hardly an expert. Isn't the top surface of alclad sheet such as RV wing skins essentially a few thousandth coating of brilliant corrosion resistant pure aluminum? If so, it seems to me that it wouldn't take much to penetrate that very soft coating of pure aluminum and that includes scuffing with maroon scotchbrite. I've always assumed something as seemingly benign as a minor scratch in alclad sheet is more than enough to expose the underlying alloy to oxygen therefore making it more vulnerable to corrosion. What complicates my understanding is that the wing ribs are alcad, yet they do not exhibit the brilliant surface coating like the wing skins do.

Frankly, I've heard so many different conflicting opinions on the subject that I don't know what to believe. Surely, there must be more than one metallurgy expert out there far more technically informed on this perennial subject than the average RV builder. Please weigh in.
 
Alclad

This is a great thread. I was told to break the shine off with a maroon scotch brite pad and then prime. I would hate to think that I was removing the Alclad but it may be so. I have asked many builders this question but the answers vary. It would be great to know for sure!
 
From Alcoa's website the alclad thickness is 5% on material thicknesses .062 thick and under. On a .030 sheet the thickness would be around 1-1/2 thousandths or .0015 per side if I understand what I read. Scuffing with a maroon Scotchbrite pad didn't change the thickness of a sheet that I measured with a micrometer. I used a dish soap and warm water wash using a scotchbrite pad as a wash cloth. The self etch primer stuck like glue.
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The main reason for scuffing is to create micro scratches that allow the primer to bond on the surface. It is in no way the intention to grind away material.
So if you just lightly scuff with maroon scotchbrite pad, you'll create very light scratches. This will not remove the alclad.

If you put force on and start really scratching, you will unnecesarilly remove part of the alclad layer.
So be very gentle when scuffing
 
rv9aviator said:
If the alclad is a few thousandths thick then you won't go through that with a maroon scotchbrite pad. Measure the thickness with a micrometer before and after light scuffing and you won't be able to measure the difference in thickness.
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I was an airframer (rivet banger) in the Navy and spent 9 years after the Navy working as a machinist in a tool and die shop. Trust me it would take some major elbow grease to wear through alclad with a maroon scotchbrite
pad. Just some food for thought I belive a beer can is approx. .003 thick (The last one I checked several years ago was) and lets say that the alclad is .002 thick. Think about how much effort it would take to rub through two thirds of a beer can with a scotchbrite pad. :eek:
 
Rick6a said:
...What complicates my understanding is that the wing ribs are alcad, yet they do not exhibit the brilliant surface coating like the wing skins do....
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The rib material starts life as 2024-0 (soft), hydroformed in that condition, then heat treated to T-4. I believe it's that heat treatment that dulls the alclad layer.

I wouldn't worry too much about scuffing off the alclad layer. If you can't feel the depth of an individual scratch with your fingernail it's less than 0.0001 (one ten-thousandth) of an inch. Just don't scuff with 80-grit!:eek:
 
If you scuff hard with the really coarse brown scotchbrite (sort of 80/100 grit) you can remove the alclad layer.

When you do you can see the sheet sort of change color as the base alloy is reached and exposed. Takes a lot of scuffing to get to this point though, so don't use lots of elbow grease and the brown scotchbrite stuff...:)
 
2024

2024 is available in both bare and alclad. The bare is actually slightly stronger than the alclad. I have always believed that the 2024 used in hydroformed parts(wing ribs, tail ribs) is typically 2024 bare, but cannot comfirm this.
From wikupedia: 2024-- due to poor corrosion resistance it is often clad with pure aluminum or AL-1Zn.
Quite a few Swifts, Cessnas, Luscombes etc out there with 60 year old polished skin. You don't have to worry about taking off the alclad with scotchbrite.
 
Take a piece of scrap aluminum with a similar size to the skins. Scuff the top 1/2 of it with scotchbright and do nothing to the other half. After cleaning, shoot your self etching primer on it. After drying and doing your "scratch test" you will see which method is best and why.
 
jrs14855 said:
I have always believed that the 2024 used in hydro-formed parts(wing ribs, tail ribs) is typically 2024 bare, but cannot confirm this.
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All 2024 sheet aluminum supplied parts in RV kits, including "formed" parts ARE alclad.
 
Yeah, and the Scotchbriting has to do with creating surface area for more bite for the primer. If you Scotchbrite, and then etch, the micro surface area created by the etch goes up exponentially.
 
This issue came up a long time ago so I went out to the garage, got some maroon pad and started scrubbing until I went through the alclad. What I learned is that there is no way in heck you could inadvertantly go through the cladding -- No Way! Incidentally, I mic'd the part as I went just to get an idea how fast I was progressing. Again, no way.
 
Alclad on the outside surface only

It is my understanding that the Alclad is only on the outside surface of the aluminum alloy stock from which Vans forms and machines the parts we receive. Thus, the inside of prepunched holes are not Alclad, they are bare aluminum alloy, and thus subject to corrosion. Also, the edges of any parts cut at the factory would also not be Alclad and likewise subject to corrosion.

If my understanding is correct, it may be that the most important priming of Alclad parts would be the rivet holes and the edges, and the surface wouldn't matter much.
 
TThurston said:
It is my understanding that the Alclad is only on the outside surface of the aluminum alloy stock from which Vans forms and machines the parts we receive. Thus, the inside of prepunched holes are not Alclad, they are bare aluminum alloy, and thus subject to corrosion. Also, the edges of any parts cut at the factory would also not be Alclad and likewise subject to corrosion.

If my understanding is correct, it may be that the most important priming of Alclad parts would be the rivet holes and the edges, and the surface wouldn't matter much.
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With all due respect, your understanding is incorrect. Both sides of an Alclad sheet are clad. It would be too difficult for a fabricator to which side was which, and too many parts would be scrapped. Also the inside of a component can corrode just as much as the outside.

It is true that the holes and cut edges have no cladding - that is why some people (who live in environments that promote corrosion) use a 'wet construction' technique - puts primer or corrosion inhibitor in every rivet hole, and between skins, before riveting.

Pete
 
Depends

The question I have is if you use a self etching primer like Vans does,
why would you want to scratch all that nice Alclad
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Not all "self etching" primers do a good job of self etching.The Sherwin Williams primer I used on my project 10 years ago is there for good. I did not scuff.

The pieces that I painted with SEM primer had to be scuffed or I could scrape it off with my fingernail.

I only have these two data points for you but personally, I would experiment (scuff vs smooth) with the product(s) you decide to use. Good luck.
 
penguin said:
With all due respect, your understanding is incorrect. Both sides of an Alclad sheet are clad. It would be too difficult for a fabricator to which side was which, and too many parts would be scrapped. Also the inside of a component can corrode just as much as the outside.

It is true that the holes and cut edges have no cladding - that is why some people (who live in environments that promote corrosion) use a 'wet construction' technique - puts primer or corrosion inhibitor in every rivet hole, and between skins, before riveting.

Pete
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I think TThurston was just trying to say that alclad is only on the surface of the metal, not through and through, and bare 2024 alloy gets exposed any time you drill or cut. I thnk the reason my 57-year old C-170 hasn't reverted to bauxite is that ANY alloy of aluminum is fairly corrosion resistant, but relatively speaking, 2024 not as much as most others.
 
longranger said:
... I thnk the reason my 57-year old C-170 hasn't reverted to bauxite is that ANY alloy of aluminum is fairly corrosion resistant, but relatively speaking, 2024 not as much as most others.
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Sorry to disagree again! It depends where you live - in the Californian desert (dry) priming is probably a waste of time. In a damp, maritime, environment 2024 will turn to white powder quickly and priming is essential.

I used to live in Texas (no point in priming), I now live in England and have seen on several occasions what water mixed with 2024 will do - for example an un-treated wing spar was scrapped due to pitting and exfoliating corrosion, even alclad will corrode readily if water is left on it for very long. I have seen the inside of a QB wing quite badly attacked - all QBs here need treating with ACF-50 (or similar) after painting.

Pete
 
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penguin said:
Sorry to disagree again! It depends where you live - in the Californian desert (dry) priming is probably a waste of time. In a damp, maritime, environment 2024 will turn to white powder quickly and priming is essential.

Pete
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Without trying to be argumentative, that doesn't explain the first 42 years of my airplane's existence in Florida, Georgia, Tennessee, and Mississippi, much of it outdoors in areas with 40-50 inches of rain per year. When I bought it in 1982 it was a ramp queen sitting outside in the Atlanta area, and you could still see your reflection in the interior sheet metal. I did have it treated once with ACF-50 (in 1991) when I moved it to coastal Mississippi. The only corrosion (surface corrsion on one fuselage bulkhead) I've ever had to address was after sitting outside here in Tehachapi for a year. Mine is only one data point, and I consider myself lucky - there are thousands of airplanes in scrap yards to contradict my experience.

And personally, I don't think where you live should even enter into the priming decision; airplanes are meant to travel.

All that said, I'm priming faying surfaces in my -7 (in addition to the 6061 stuff of course), but it's awful tempting not to.
 
About Time!!!!

McFly said:
Not all "self etching" primers do a good job of self etching.The Sherwin Williams primer I used on my project 10 years ago is there for good. I did not scuff.

The pieces that I painted with SEM primer had to be scuffed or I could scrape it off with my fingernail.
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Hugh

I have been reading through all the "Primer Wars" and this post is the most definitive post I have read. 10 years and on there for good! That is what I am looking for!!! Exactly, which Sherwin Williams did you use??? I plan on doing a wet build myself....
 
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