Urathane over Alodine

Vans builders guide offers the example of their early -4 that was acid etched, alodined and painted with Imron. No primer. Going for as little weight as possible.

A call to Dupont tech support yesterday gave us instructions not to paint over a conversion coating (Alodine) because the acid will leach out under the urathane coat and possibly blister, peel, etc.

Anyone have any experience with this process? Or do we know how long the -4 paint job lasted?

A few things to consider...

It has been nearly 30 years since that process was done on the prototype RV-4.

Paint formulations have changed a huge amount since then.

That information will be removed from Section 5 during the rewrite that is going on right now.

With the formulation changes that have occurred in paints, I would be even more concerned about adhesion to the substrate than I would have 30 years ago.

To anyone serious about a light weight paint finish, I would recommend the following...

Stay away from 2 stage systems (base coat / clear coat)

Use a paint where the recommended primer can be shot as a sealer (reduced further so that it can be sprayed on thin), and can be tinted.

Have the primer tinted to approximate the base color you will be spraying over it (to minimize the amount of finish coat needed for coverage).

Use a high solids paint system (takes less paint for full coverage) and choose colors that will cover well with minimal coating buildup.

Good point. I hadn't considered the time lapse and evolution of paint formulas since.

I'll be doing my own paint and know that every coat is a opportunity for a mistake that needs to be sanded out. On auto bodys that doesn't bother me. Over 1000's of rivets, I'm worried.

I agree with single stage. Not building a show plane. Dupont still wants a seal coat over the etching primer. Thats 3 times the chance for a mess.

Blain said:

Good point. I hadn't considered the time lapse and evolution of paint formulas since.

I'll be doing my own paint and know that every coat is a opportunity for a mistake that needs to be sanded out. On auto bodys that doesn't bother me. Over 1000's of rivets, I'm worried.

I agree with single stage. Not building a show plane. Dupont still wants a seal coat over the etching primer. Thats 3 times the chance for a mess.

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That's why I don't use etching primers for exterior finishing.

Most painters scotchbrite scuff the entire exterior (surface abrades way better than any etching chemical process can).

I do the same, and then when ready to prime/paint, wipe down with a etch/cleaner, wipe down with a surface prep (acryliclean or similar) and then prime ASAP (within an hour) to minimize the chance for surface oxidation to begin again. The reason for this is because I skip the traditional conversion coating process. It is very toxic, and there is no way to do it without using water to rinse. I believe that leaves a very high risk for water entrapment which will cause filliform corrosion later (unless you have a heated drier you can put your parts in for a while... which few of us have access too).

Along with helping prevent corrosion, alodine helps the primer stick. Paint likes to stick to primer. Painting without primer will lead to paint failure. The weight you will save will be very small. Maybe even to small to notice. For an RV-4 I would mix up about 2 quarts of primer to prime ALL metal surfaces. With the transfer efficiency of the spray gear there is maybe 1.5 pints actually on the metal. Going to the bathroom before flying will save just as much weight.

KTM520guy said:

Along with helping prevent corrosion, alodine helps the primer stick. Paint likes to stick to primer. Painting without primer will lead to paint failure. The weight you will save will be very small. Maybe even to small to notice. For an RV-4 I would mix up about 2 quarts of primer to prime ALL metal surfaces. With the transfer efficiency of the spray gear there is maybe 1.5 pints actually on the metal. Going to the bathroom before flying will save just as much weight.

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Aviation Plasticized Pigmented Polymer Application Engineer
I'm going to guess and say you do this for a living? So, after surface abrasion, which primer do you use?

I use a Sherwin Williams high solids epoxy primer from their military line. The price is very reasonable and one can (plus hardener and reducer) is enough to prime two small RV's. RV-10's need just a little more. I'm not sure of the part number right now. I'll go look when I get a chance.

[QUOTE
Stay away from 2 stage systems (base coat / clear coat)
][/QUOTE]

Scott, can you give the reason you don't like base/clear. I have a friend who used polyurethane enamel (Imron) on his plane. It looked great but now he has to repaint some hangar rash. He can't blend or match the product.

Thanks, Mark

Maxrate said:

[QUOTE
Stay away from 2 stage systems (base coat / clear coat)
]

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Scott, can you give the reason you don't like base/clear. I have a friend who used polyurethane enamel (Imron) on his plane. It looked great but now he has to repaint some hangar rash. He can't blend or match the product.

Thanks, Mark[/QUOTE]

What I actually said was ...
To anyone serious about a light weight paint finish, I would recommend the following...

Stay away from 2 stage systems (base coat / clear coat)

To a large degree 2 stage finish systems were designed to simplify field repairs (body shops, etc.).

There is nothing wrong with 2 stage on aircraft but I would never use them unless my goal was to build a trophy winning show plane.
They are more expensive, more work to apply, and in general are heavier (though that is still dependent on the skills and application choices made).

The thing that bothers me the most though (in my opinion) is that they also aren't as abrasion and chip resistant, or have as good of a resistance to damage by chemical exposure. This is in comparison to the polyurethanes that are popular for aircraft use (Imron, Jet Glo, etc.).

This comment is somewhat subject to how high a quality clear you are using, because there is a wide range available (using an inexpensive clear is not even worth the trouble, you definitely get what you pay for).

In my opinion, Single stage polyurethanes are the best overall paint choice (there is a reason private jets, etc., are not generally painted with 2 stage paint systems. I admit you are trading repairablity, and 2 stage is usually more forgiving for use by amateurs (fixing mess ups, etc.), but single stage would still be my personal choice.

Are you sealing after the epoxy or shooting urathane direct?

My RV-6 was painted with single-stage Jet-Glo in 1993 and still looks great today.

Blain said:

Are you sealing after the epoxy or shooting urathane direct?

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I very lightly scuff my primer then paint directly to it. No sealers are needed with aviation specific paint systems.

I can't speak to the change in paint formulations over the years, but I painted my Pitts in 1980 with alodine for all al. parts, followed by Corlar primer, then Imron. After 25 years of fun-fun-fun, it now resides in our local air museum. There has been no shedding of paint on either the metal or fabric, and the paint still has the same wet look as on day one. It was not the lightest paint job, but it looked great.

Jim Berry
RV-10

Scott,

Thanks for the insight. back in the eighties when I was in high school I did a lot of paint work using either acrylic enamel or lacquer. Base clear is a totally new product on the market since then. The urethanes seem to flow like the acrylic enamels but are much tougher and don't fade. Definetly a superior choice for aircraft. I guess you could compare the base clear to lacquer as far as a labor intensive time element, and more equiptment to purchase to do the job.

Again thanks for the input.

Blain said:

Are you sealing after the epoxy or shooting urathane direct?

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The only time I see sealing before top coating recommended, is when doing repairs and painting over unknown paint finishes. Sealing (with an extra thinned coat of primer) is used to prevent the possibility of bleed through and to assure good adhesion to the original finish.

All the paint systems I have used allow for painting directly on the epoxy primer with no other work steps done between, as long as it is done within a specified maximum re-coat time. This is to assure good cross linking between the paint and primer. If you wait too long, the catalyzed primer cures to much to allow the solvents in the paint to dissolve the surface and cross link the paint to the primer (to prevent delamination of the paint).
This is why priming major assemblies as you finish them is counter productive.
All those parts that were primed and then fully cured will have to be surface sanded and then re-primed get the full level of cross link for good paint adhesion.

The only system I have used that wanted to be reprimed was Courtaulds (bought out by PPG in 2000). With U.S. Paint Alumi-Grip (now Akzo Nobel) and Jet-Glo (Sherwin Williams) just sand your primer with 320 sand paper, tack, and spray.

The epoxy I've used on autos is a high build. Don't need to cover imperfections. I've never tried to thin out epoxy primer. How light can it be applied?

Blain said:

The epoxy I've used on autos is a high build. Don't need to cover imperfections. I've never tried to thin out epoxy primer. How light can it be applied?

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I don't know, and it may depend on the product you are using.

The primer I use a lot specifies different reduction whether you are using it as a normal high solids primer or as a sealer.

rvbuilder2002 said:

Stay away from 2 stage systems (base coat / clear coat)

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My RV-6 (purchased already complete, painted and flying) was painted with clearcoat over polyurethane. Last summer, an overfilled fuel tank while parked in a hot hangar oozed 100LL thru the fuel cap and left a nasty blue stain all around the fuel cap in the white paint on top of the fuel tank/wing. The stain got thru the clearcoat and into the white underneath. After a year, it has still not faded away completely. I tried various solvents and even some polishing compound and almost went thru the clearcoat in a couple spots trying to get rid of the stain before I gave up.

My old Cherokee 140 was painted with single stage Sherwin Williams Acry-Glo "Marathon White". One of the fuel drains dripped onto the wheel pant for a few days in the hot hangar. Left a really nasty blue stain in the white paint all over the wheel pant. A little bit of polishing compound and a little elbow grease *completely* removed the stain in minutes.

If I ever have to repaint my -6, it'll be with single stage polyurethane for certain.