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Possible priming alternative.

GPV

Member
Hi All,

I have been trying to find a suitable solution to the priming issue that is safe, effective and consumes minimal time. I believe I may have a reasonable compromise, provided I'm not missing something (there's a few things I am unsure about, see below). I'd be interested to hear your thoughts. First, a high level summary of the available options used to date.

1) Prime with a chromate based primer (+/- Alodine)
Pros: Most effective anti-corrosion option.
Cons: Time consuming, hard work, toxic. Removes protective alclad layer (if scotch brite used rather than alodine). Expensive.

2) Prime with rattle can primer.
Pros: Convenient. May work.
Cons: Still a fair bit of work and somewhat toxic. Removes protective alclad layer (if used with scotch brite). Unless using chromate-based product, it offers minimal cathodic protection and relies on providing a weatherproof barrier of varying integrity and adhesion. Expensive. Susceptible to damage while constructing.

3) Prime with 2-pack, non-chromate primer
Pros: Less toxic than chromate options. Probably reasonably effective.
Cons: Lots of work, pretty toxic, inconvenient.

4) Don't prime, treat with Corrosion-X or similar.
Pros: Very convenient. Preserves protective alclad. Plane gets built in much less time!
Cons: Messy for years afterwards (apparently), relies on accessing all parts of the interior airframe with a spray. Not 100% familiar with the product but understand it remains a liquid.

So, what if there was something that provided a solid robust barrier like epoxy, was as convenient as Corrosion-X, non-toxic and didn't remove the alclad? I think there might be.

I want to present a product I have used in automotive restorations for years - Penetrol. If you have a sill that is rusting out, this is the best way to stop it in its tracks. It's like a 21st century version of fish-oil, but stops smelling, sets hard and is more robust.

You can find info about this product here:

http://www.floodaustralia.net/products/anti_corrosion/penetrol-anti_rust.php

This product starts out as a relatively low toxicity, super thin oil that can penetrate anything like WD40 spray. Seriously, it finds its way into every crevice. It will easily work its way under ribs and skins, if used as I outline below. Then, after application, it slowly hardens into a very tough clear flexible film and stays this way up to 100 degC (very hot). It's so flexible that the manufacturer has videos of it sticking to plastic bags. However, it is also easily as hard and adhesive as rattle can primer, probably more. So we have something like corrosion-X, which after being applied goes hard and is reasonably permanent. So it won't keep running out of the cracks for years afterwards and spoiling your paint. You have the protection of a basic primer, without removing the alclad, and almost no preparation. It will also fill tight crevices and prevent water from working its way in. One thing I am unsure about is how well it will stick to Aluminium, but I think it will be ok (going to do a test soon).

I imagine a treatment workflow going like this.

1) Clean parts with acetone or alcohol to remove oil.
2) Completely assemble component, e.g. rudder.
3) Treat insides with excessive amounts of Penetrol using spray, rotating part in all orientations to ensure spray gets into all crevices and onto all surfaces. Collect run-off in container to use another time.
4) Hang part up length ways with bucket to collect drips.
5) Once in a while over the next few days, rotate part to prevent pooling in any internal location, remove any penetrol that finds its way onto the outside using a rag. It is important to rotate regularly at the start while the oil is still thin enough to work its way out.
6) Leave for extended period to harden once dripping has stopped.

Of course this assumes it will stick to Aluminium. If not, I am sure using phosphoric acid to etch it would be a suitable treatment. The Manufacturer does say it is ok on non-ferrous metals.

Also, some parts could be lightly coated while assembling if desired. It comes in spray cans and 1L tins.

In line with above, let's discuss the pros and cons:

5) Treat with Penetrol
Pros: Super easy, should be effective in providing permanent weatherproof barrier to prevent corrosion. Preserves Alclad. Low toxicity.
Cons: Flammability - does it matter? Weight - should be ok but unsure, will depend partly on how carefully excess is removed while it's wet. No heavy pigments at least. No cathodic protection. Will it stick to Aluminium ok?

I reckon this treatment might offer a great compromise to the priming issue, unless some of the factors I outline in the cons section come in to play. Definitely not sure how thick and heavy it will end up being, but hopefully less than epoxy??

Keen to hear your thoughts, I'm sure someone has come up with similar things before.

Greg
 
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As far as I know Alumiprep + Alodine used per directions does not REMOVE the alclad layer. The Alumiprep will etch the alclad (or the aluminum oxide layer on non alclad aluminum such as extrusions) and the Alodine will do a chromic conversion. Both only change the top few microns of the surface.

Unless I misunderstand your statement, chromate based primer also does not REMOVE the alclad layer.

CorrosionX or Ardrox AV8 or similar should be used after the airplane is externally painted as it will migrate through the seams and make it really hard to paint properly.
 
As far as I know Alumiprep + Alodine used per directions does not REMOVE the alclad layer. The Alumiprep will etch the alclad (or the aluminum oxide layer on non alclad aluminum such as extrusions) and the Alodine will do a chromic conversion. Both only change the top few microns of the surface

Yeah I should have been clear that in that case I was referring to the use of roughing with scotch brite and priming directly which seems pretty common these days.

Not sure why this treatment would need to be done after painting. It sets hard and will not weep out once set. Even when set it is easily removed with the right solvent.
 
When I painted my 6, I planned to sand with 180 prior to epoxy primer. I wasn't really worried about the alclad layer, as the epoxy primer would be the new barrier. However, I ran a test. I hit a test sheet with 120 grit on a DA sander. It literally took a few minutes with that set up to break through the alclad layer in the small test area (you can seen the color change when you go through). I can assure you that scuffing with a scotch pad will not remove the alclad layer. It is not like electro plated gold that is measured in microns. It was well over a thou. Further, there is no concern with the alclad layer if you are applying primer to protect the piece. If you are truly concerned about this, how do you feel about all of the non clad alum in your plane.

If I remember the spec, the alclad layer is a thou and half. I dare you to do an experiment. Get your mic or calipar out and keep working a piece with your scotchpad until you remove a thou and a half. I promise you that your arm will fall off before getting there.

Larry
 
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Ok i clearly didn't research enough. This contradicts the few posts I read on the matter.

Looks like I'm back to priming then!
 
Hi Greg,

There are a few other options for you.
I used Stuart Systems Ekopoxy on the majority of my skins. Uses water as a carrier, so it's not nearly as toxic to lay down, however it is quite a bit more sensitive to having the right conditions to spray, temperature and humidity wise, compared to the alternatives.
For the smaller parts, I and many people in Australia use zinc rich self etch single pack epoxy primer (Valspar/Wattyl Superetch). It's still pretty durable, but not to the level of the two pack, but it is easy to work with. You can lay it down in a wide range of temperature/humidity conditions, and you can get it in cans or bulk. I used both but do admit that the ability to use cans at the end was great, as toward the end of the project you seem to be doing lots of small projects where you need to prime half a dozen small parts. I used the 3M PPS/Dekups system with my paint gun, but still it's so nice not having to clean up after rattle can priming.
As a final option, you can fay seal the mating surfaces and forget about priming the skins themselves (you still need to prime the non-alclad parts). Kind of like wet riveting a fuel tank. The worst corrosion in aircraft is generally between faying surfaces, so if you can seal these with a compatible sealant of some type when you are riveting them together, it stops the moisture from getting in there in the future. It actually weighs very little (less than priming), as most of it is extruded out of the joint and cleaned off wet. I did the lower half of my fuselage (one of the most susceptible regions to corrosion) with sikaflex pro polyurethane, however if I could do it again I'd think hard about doing the entire aircraft with a modified sili polymer. These are similar in performance to polyurethane but way less toxic (I wore a respirator and gloves when wet riveting the fuselage). I just couldn't find one with a long enough working time, but I believe they are out there.
That said, I'm glad I went the whole hog with priming and if I had to do it again I may even consider Akzonobel two pack for the extra durability, you've just got to be prepared with the extra time it takes to deal with two pack (not insubstantial), but at the end of the day, if you handle the etch primer right, it's still pretty resilient. In the last week I've had to fly to two islands for work and after just a day on the coast you can see salt all over your aircraft, which is the moment you realize that it was worth the effort to do things properly.

Tom.
RV-7
 
Thanks Tom,

Yeah I am now tossing up between the ekopoxy and going a full blown chromate epoxy primer. I'm not overly impressed with the performance of the super etch, I was able to scrape off huge strips to shiny metal with a hard bit of plastic. In addition, a local manufacturer informs me that zinc phosphate provides little to no cathodic protection for aluminium (if you have some data that proves otherwise let me know!). Apparently it's really only good for protecting iron, so that part of it is wasted. When I tried it I noticed that it also has a porous appearance so I'm not even sure it provides good isolation from the elements without a topcoat. I suspect if one chooses to avoid chromates than a simple passive epoxy like ekopoxy or similar is the way to go.

Agree fay sealing sounds like a good idea but the process you described with gloves and a respirator sounds terrible! Neutral cure silicone might be a good option... Hmm...
 
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So what about the SW wash primer? No one is bringing it up. Did I miss something? is it not useful?
 
Hi Aviator,

We're not ignoring Sherwin Williams Wash Primer, as it is undoubtedly gets the job done, as Van's have proven in their "under the stairs" corrosion test and uses it in their quick build kits for a reason. It's just that we were exploring alternatives to using chromate primers, as this is what P60G2 is classified as.
https://www.paintdocs.com/docs/webPDF.jsp?SITEID=SWOEM&lang=E&doctype=SDS&prodno=P60G2
https://toxnet.nlm.nih.gov/cgi-bin/sis/search/a?dbs+hsdb:@term+@DOCNO+6188

To Greg,

Not all zinc etch primers are created equal. I now see you are based in Australia, so Wattyl Superetch should be available where you are. I've tried a variety of etch primers, and this is the best from a durability and adhesion perspective. From a corrosion perspective, I have not tested it personally, however Wattyl is a Valspar company, and their zinc phosphate etch sold in the US meets the 800 hour 5% salt spray test. If I was a gambling man, I'd say this stuff is identical to what they sell here as Superetch.

https://www.aircraftspruce.com/catalog/cspages/zinc.php
https://wattylindustrial.com.au/documents/industrial/tds/metal/Super Etch.pdf

That said, I'm not the first person to use Superetch in Australia, so I'm sure there are plenty out there with long term experience with these primers, so hopefully other Australians can chime in, as most of us live in moderate to high corrosion environments near the coast. I did speak to a builder that completed his RV-7 about 15 years ago in Melbourne using zinc phosphate etch primer and said he had observed no corrosion.

On the fay sealing, it's really not that bad. I spent most of my time building in a half faced respirator (3M 7502) and if you get the light weight 3M 2297 filters, it's the most comfortable setup you will ever wear. Any cutting, edge finishing, solvent work etc. I'm in it, so literally, it was probably half the buld. I use the 3M 60923 filters when spraying primer outdoors (or my Hobbyair system when doing large quantities or using two pack polyurethane). The same applies to Nitrile Gloves. Find a quality pair that fit well, and you'll be set. Quality ones can be re-used multiple times (I just blow them back the right way with a quick bust of air from the compressor). You will be using solvents a lot, and I know too many LAME's and mechanics that have developed cancers attributed to excessive hydrocarbon exposures.

The most important thing is though, don't use any silicones near your aircraft if you can help it (apart from 3M Firebarrier 2000+ when sealing the firewall) due to the problems it can cause later when you are trying to paint. This is not an issue with the polyurethane and to a certain extent the modified silicone polymers. Most of the MS polymers should be paintable, but I would test any before using them in this manner, as you'd hate to end up having a botched paint job because of it. The other benefit of the MS polymers is that they don't offgas like the polyurethanes, making them far less toxic to work with.

Anyway, I hope this helps.

Good luck.

Tom.
 
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Just a word here on priming.....You guys are going WAY overboard on scuffing prior to priming. You don't need sandpaper, especially 120 grit. Just use red Scotchbrite. All you need to do is break the glaze. This will NOT remove the alclad. 2024 aluminum itself is VERY corrosive. That's why it is normally alclad. I REPEAT....Do NOT remove the alclad!
 
Just a word here on priming.....You guys are going WAY overboard on scuffing prior to priming. You don't need sandpaper, especially 120 grit. Just use red Scotchbrite. All you need to do is break the glaze. This will NOT remove the alclad. 2024 aluminum itself is VERY corrosive. That's why it is normally alclad. I REPEAT....Do NOT remove the alclad!

I agressively abraded the exterior during paint to get a better mechanical adhesion and my primer stuck like glue. This was based upon the recommendation from a paint manufacturer. the more agressive the grit, the better the bond, assuming the paint can still leave a smooth finish. It should be noted that I did this with a DA sander and 180 on a DA leaves a finish similar to 220.

I wasn't recommending this approach for others, only passing on a relevant data point about alclad thickness.

Larry
 
aluminum

To say that 2024 aluminum is "VERY corrosive" may be a bit of an embellishment...

Is the 2XXX series more prone to corrosion than other alloys? Yes, but you have to put that in perspective.


Here is a reference concerning corrosion of aluminum...probably more than you ever wanted to know...

https://www.arl.army.mil/arlreports/2009/ARL-TR-4937.pdf

There are numerous other references to cite, as well...Here is an excerpt from an ASM International text titled "Corrosion of Aluminum and Aluminum Alloys":

"Corrosion Resistance. When aluminum surfaces are exposed to the atmosphere, a thin invisible oxide skin forms immediately, which protects the metal from further oxidation. This set protecting characteristic gives aluminum its high resistance to corrosion. Unless exposed to some substance or condition that destroys this protective oxide coating, the metal remains fully protected against corrosion. Aluminum's highly resistant to weathering, even in industrial atmospheres that often corrode other metals..."

and...

"Aerospace. Aluminum is used in virtually all segments of the aircraft, missile, and spacecraft industry--in airframes, engines, accessories, and tankage for liquid fuel and oxidizers. Aluminum is is widely used because of its high strength to density ratio, corrosion resistance, an weight efficiency, especially in compressive designs.
Increased resistance to corrosion is secured through the use of alclad alloys or anodic coatings. The exterior or aircraft exposed to saltwater environment is usually fabricated from clad alloys. Anodized bare stock successfully resists corrosion when only occasional exposure to saltwater is encountered. Corrosion resistance can further be enhanced by organic finishes or other protective coatings..."
 
To say that 2024 aluminum is "VERY corrosive" may be a bit of an embellishment...
Is the 2XXX series more prone to corrosion than other alloys? Yes, but you have to put that in perspective.

When I said that 2024 is very corrosive, I was implying that it is more corrosive than other aircraft aluminums such as 6061. 6061 is not quite as strong as 2024, but it is much more corrosive resistant.
 
2024 aluminum itself is VERY corrosive. That's why it is normally alclad. I REPEAT....Do NOT remove the alclad!

If this were true, all of the unprimed kits would be rotting to pieces, as there are MANY parts of the plane made from non-clad 2024. It's true that 2024 is less corrossion resistant than pure Al. It is inappropriate to say that it is VERY corrossive. This simply isn't true. Leave a piece of 4130 outside for the summer next to a piece of 2024. Night and day difference in corrossion. It would be fair to say the 4130 is very corrossive. My 10 kit is 13 years old and not a spec of visible corrossion on any 2024 part.

Larry
 
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If this were true, all of the unprimed kits would be rotting to pieces, as there are MANY parts of the plane made from non-clad 2024. It's true that 2024 is less corrossion resistant than pure Al. It is inappropriate to say that it is VERY corrossive. This simply isn't true. Leave a piece of 4130 outside for the summer next to a piece of 2024. Night and day difference in corrossion. It would be fair to say the 4130 is very corrossive. My 10 kit is 13 years old and not a spec of visible corrossion on any 2024 part.
Larry

There are ver few, if any, unclad 2024 parts in the RV kits. The unclad parts are 6061.
Your comparison of aluminum to steel is not exactly apples to apples.
 
But

...but comparison of steel, which is very susceptible to corrosion, and aluminum which is not very susceptible to corrosion, is a good illustration of perspective.
 
OK, My original post was to recommend that you not sand off the alclad before priming.
If you guys want to do that, go right ahead. I'm out-a-here.
 
To Greg....
Good luck.

Thanks Tom that was some really sound advice and I think you have sold me on the superetch. I went back to my test part and did a few more scratch tests and was unable to reproduce my previous results with plastic, except in places I had perhaps gone a bit thick on the coats. So I'm reasonably convinced it will cling on for a few years.

Looking at the valspar data sheet you linked I see the product does not need top coating either, and as you say at least meets some form of corrosion standards. I think I'm reasonably happy to proceed with it. Very glad to be here as the prospect of mixing 2 pack and cleaning a gun all the time was getting very discouraging.

I have a 3M respirator also and note all of your suggestions.

Thanks again,

Greg
 
OK, My original post was to recommend that you not sand off the alclad before priming.
If you guys want to do that, go right ahead. I'm out-a-here.

The discussion was around sanding for the application of a sealing primer, which provides the same corrosion protection as the aluminum cladding and in many cases does so more effectively. What is your reasoning for preserving the alum cladding in this case? I suppose it is a good backup in case the primer chips off, but can't think of any other reasons.

Larry
 
You are right in that it becomes an additional form of protection and this is what people near the coast are looking for, as many of us operate in severe corrosion conditions. It's not done just for fun.
I suspect Mel's real hangup on using 120 grit is that Van's stipulates in the manual that everything needs to be finished to a 600 grit finish to prevent crack initiation/propagation. Section 5.2 of the manual spells all this out, and based on my experience and that of many others, more than maroon scotch brite for surface prep prior to paint application is completely unnecessary. It is more important to make sure the surface is completely clean (I perform multiple cleanings using automotive wax and grease remover) prior to primer application.

Tom.

An extract from the manual is as follows:
"The finishing procedures just described will constitute a sizable portion of the total building time. However, they are important for structural reasons as well as cosmetic. Most of these holes, edges, etc. will be inside the airframe and out of sight when the airplane is finished. This is no reason to consider them unimportant. The need for good edge finishing is most difficult to impress on new builders unaccustomed to aircraft standards.
Scratches in the surface of aluminum can have the same weakening effects as rough edges, corners and holes. The alclad sheet used is very easily scratched because of the thin surface layer of soft aluminum. Scratches within this layer will have little effect on strength, but deeper scratches will. The greatest difficulty is deciding how deep a scratch can be before it is a potential problem. The best approach is taking extra care to prevent scratches in the first place. When a scratch does occur sand or buff it out no matter how small. Very light scratches can be removed with #600 wet sandpaper. Deeper ones will require #400 (or perhaps more coarse) sandpaper, followed by #600 for finishing. One thing to remember when removing scratches is that in doing so the corrosion resistant alclad surface of the aluminum is also removed. Therefore any area that has been sanded for scratch removal must be primed"
 
You are right in that it becomes an additional form of protection and this is what people near the coast are looking for, as many of us operate in severe corrosion conditions. It's not done just for fun.
I suspect Mel's real hangup on using 120 grit is that Van's stipulates in the manual that everything needs to be finished to a 600 grit finish to prevent crack initiation/propagation. Section 5.2 of the manual spells all this out, and based on my experience and that of many others, more than maroon scotch brite for surface prep prior to paint application is completely unnecessary. It is more important to make sure the surface is completely clean (I perform multiple cleanings using automotive wax and grease remover) prior to primer application.

Tom.

An extract from the manual is as follows:
"The finishing procedures just described will constitute a sizable portion of the total building time. However, they are important for structural reasons as well as cosmetic. Most of these holes, edges, etc. will be inside the airframe and out of sight when the airplane is finished. This is no reason to consider them unimportant. The need for good edge finishing is most difficult to impress on new builders unaccustomed to aircraft standards.
Scratches in the surface of aluminum can have the same weakening effects as rough edges, corners and holes. The alclad sheet used is very easily scratched because of the thin surface layer of soft aluminum. Scratches within this layer will have little effect on strength, but deeper scratches will. The greatest difficulty is deciding how deep a scratch can be before it is a potential problem. The best approach is taking extra care to prevent scratches in the first place. When a scratch does occur sand or buff it out no matter how small. Very light scratches can be removed with #600 wet sandpaper. Deeper ones will require #400 (or perhaps more coarse) sandpaper, followed by #600 for finishing. One thing to remember when removing scratches is that in doing so the corrosion resistant alclad surface of the aluminum is also removed. Therefore any area that has been sanded for scratch removal must be primed"

Thanks. Per the quote, "scratches within this layer [alclad] have little effect on strength." The alclad layer is at least .001" thick and my sanding put scratches in with a depth of way less than 1/4 of that, so I have no concern about strength. As mentioned, I had to sand in one spot with power tools for several minutes to get through the alclad layer. Also, I used 180 grit for exterior paint prep. I just used the 120 for the test to see how fast it could go through the alclad.

Again, I am not advocating or recommending my approach. I was concerned about adhesion and took steps to address it.

Larry
 
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...

An extract from the manual is as follows:
"The finishing procedures just described will constitute a sizable portion of the total building time. However, they are important for structural reasons as well as cosmetic. Most of these holes, edges, etc. will be inside the airframe and out of sight when the airplane is finished. This is no reason to consider them unimportant. The need for good edge finishing is most difficult to impress on new builders unaccustomed to aircraft standards.
Scratches in the surface of aluminum can have the same weakening effects as rough edges, corners and holes. The alclad sheet used is very easily scratched because of the thin surface layer of soft aluminum. Scratches within this layer will have little effect on strength, but deeper scratches will. The greatest difficulty is deciding how deep a scratch can be before it is a potential problem. The best approach is taking extra care to prevent scratches in the first place. When a scratch does occur sand or buff it out no matter how small. Very light scratches can be removed with #600 wet sandpaper. Deeper ones will require #400 (or perhaps more coarse) sandpaper, followed by #600 for finishing. One thing to remember when removing scratches is that in doing so the corrosion resistant alclad surface of the aluminum is also removed. Therefore any area that has been sanded for scratch removal must be primed"

That's a bit crazy, IMHO.

I've worked on many many aluminum aircraft over the years, of all types. I've de-riveted, fabricated new parts and re-riveted them back together along with existing products. I've seen the insides of all brands of aluminum aircraft, including those with and without previously-applied primer. I've NEVER seen aluminum components buffed/sanded/polished to that degree. Not once that I can remember.

My empennage kit has not yet arrived, so I haven't read that section of the instructions for myself. So what's the rationale for that level of smoothing (polishing) before assembly? I understand the whole "stress riser" deal, but I think that's extreme overkill...based upon several years' experience working as an A&P. Maybe someone smarter than I can enlighten me though.

As to ALCLAD, I seem to remember we were taught in A&P school that it was a layer of 1100 aluminum several thousandths thick. I've primed a bunch of parts in my life (lost count in the 1980s), and have never done anything other than some light action with medium (red) Scotchbrite. There are very light scratches visible on the surface but I've never gotten anywhere close to removing the ALCLAD layer--at least that I was able to tell. In the 80s' & 90s we would always just use a Zinc Chromate sprayed primer (can or gun), and it would suffice just fine. I owned a 1967 Piper Aztec and re-primed the inside of the thing in 2001, some 33-34 years after the thing was primed to start with. The only reason it needed it was because I had to use lacquer thinner to clean the inner skin surfaces, and that took the primer off quite readily.

So call me crazy, but I am not sure why such extreme polishing needs to be done to avoid "stress risers." I can assure you that the 50-60+ year-old Piper and Cessna aircraft that we see flying every day weren't treated to that extent. I've worked on many of them, and didn't find one whose parts had been polished with 600-grit sandpaper.

As for priming, I will be doing it as much as to facilitate easier inspection of internal structures as anything. I mean, sure...the corrosion resistance is great. But the real reason I like it is because it makes it a virtual pleasure to inspect interior structures when they've been primed: It's easier to see abnormalities when you don't have a bright LED light reflecting in all directions off a million different smooth aluminum surfaces. Been there, done that.

Interesting discussion though, as I'm looking for a primer product to use here in a few weeks.

TB

EDIT: Re-reading the verbiage that Tom quoted from the manual, I guess I can maybe see why there are saying this. It's certainly massive overkill, but they of course have to give some guidance to "amateur" aircraft builders. So call it "CYA," or call it "providing explicit guidance to inexperienced builders," and I guess it's all the same. And I do agree with their recommendation about "deep scratches" (I'd call them gouges) weakening the structural integrity of the aluminum skin. But I do see some folks taking things to extreme on YouTube, when they are documenting the preparation they put into parts prior to riveting things together--and that's fine, as long as they're happy with the results and can feel that they've done their best during the assembly process.
 
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