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  #1  
Old 08-06-2019, 09:38 PM
GPV GPV is offline
 
Join Date: Apr 2019
Location: QLD, Australia
Posts: 45
Default Possible priming alternative.

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

Last edited by GPV : 08-06-2019 at 10:05 PM.
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  #2  
Old 08-06-2019, 09:54 PM
terrye terrye is offline
 
Join Date: Feb 2010
Location: Vancouver, BC, Canada
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Default Possible priming alternative.

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.
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  #3  
Old 08-06-2019, 10:02 PM
GPV GPV is offline
 
Join Date: Apr 2019
Location: QLD, Australia
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Quote:
Originally Posted by terrye View Post
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.
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  #4  
Old 08-06-2019, 10:37 PM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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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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Last edited by lr172 : 08-06-2019 at 10:44 PM.
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  #5  
Old 08-06-2019, 11:32 PM
GPV GPV is offline
 
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Location: QLD, Australia
Posts: 45
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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!
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  #6  
Old 08-07-2019, 12:11 AM
tgmillso tgmillso is offline
 
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Location: Launceston, Tasmania, Australia
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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
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  #7  
Old 08-07-2019, 02:47 AM
GPV GPV is offline
 
Join Date: Apr 2019
Location: QLD, Australia
Posts: 45
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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...

Last edited by GPV : 08-07-2019 at 02:55 AM.
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  #8  
Old 08-07-2019, 04:58 AM
Aviator Aviator is offline
 
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Location: Hixson, TN ( Chattanooga)
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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?
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  #9  
Old 08-07-2019, 06:41 AM
tgmillso tgmillso is offline
 
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Location: Launceston, Tasmania, Australia
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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/webPD...S&prodno=P60G2
https://toxnet.nlm.nih.gov/cgi-bin/s...rm+@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/docu...per%20Etch.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.

Last edited by tgmillso : 08-07-2019 at 06:47 AM.
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  #10  
Old 08-07-2019, 07:47 AM
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Snowflake Snowflake is offline
 
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Quote:
Originally Posted by GPV View Post
5) Treat with Penetrol
Cons: Flammability - does it matter?
Not until you're on fire, and would literally give your life to make it stop.
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