Peeling Paint?

I got a question the other day from reader Dale Thomsen. He reports that:

“I got a Stinson SR-9 from another modeler. My problem is the paint that was on the plane is pealing off. I’m sure it is Stits covering and don’t know what to do. the paint is Sig Supercoat if this helps. I’ve never worked with Stits before. Will I have to remove all the covering and start over? Or do you have any help for me? I could keep doing touch ups as it comes off.”

Dale, I guessing that your problem is one we can solve, but it may take a little work. If your covering is indeed unsealed white polyester fabric (without the adhesive coat you find on Coverite, Solartex, etc.), it is probably either Stits PolyFiber or Sig Koverall. For what I’m suggesting, either is OK.

The problem is that Sig Supercoat is butyrate (cellulose acetate butyrate) dope. It has been a favorite of model builders for generations because it’s “fuel proof”. That’s fine on any of the traditional coverings, but butyrate won’t stick reliably to polyester fabric. It seems to go on and dry OK, but eventually begins to peel, as you found out. The solution (and this goes for full scale practice as well) is to use nitrate (cellulose nitrate) dope as your sealing and sanding base coats over polyester cloth. Nitrate bonds well with any tissue or fabric you can think of, and just about any possible model finish…more nitrate dope, butyrate dope, epoxy, lacquer, enamel, acrylic…sticks to the nitrate. Butyrate dope does not like polyester, and the only coating that seems to stick well to butyrate dope is more butyrate dope.

Traditionally the reason nitrate has not been very popular with model builders is that it is not fuel proof when used with alcohol based fuels. If you’re flying electric that doesn’t matter, but for a long time that wasn’t an option and we all got used to needing fuel proof (butyrate) dope. When iron-on finishes came on the scene, more and more modelers stopped using dope altogether , it became less and less common on the hobby shop shelves, and over the years nitrate dope got overlooked and became hard to find in the RC model airplane world. It never really went away, though. In your case, good old nitrate is just what you need to solve the problem.

What has to happen is that you have to get rid of the butyrate dope. By definition that job has been started for you. Peel off as much of what is still attached as you can…the underlying fabric won’t be harmed. You’re not going to get it all, though, so the next step (which gets messy) is to strip all remaining butyrate with a solvent. In my experience acetone or MEK (methyl ethyl ketone) works best. You’ll need a lot – probably at least a gallon – and you must wear protective gloves and have good ventilation whenever you’re working with either of those solvents. The only downside (beyond the mess and the patience required) is that either solvent will soften the fabric adhesive if you let the bonding areas get too wet. Sneak up and the leading and trailing edges and so on, be careful, and just re-attach any loose covering edges that might have happened once you’re done stripping.

I haven’t had to strip a butyrate finish recently so I don’t have any photos of what the job should look like. The best I can do is offer this shot of the main characters in the story. The nitrate clear dope sold by Sig works just fine for jobs like this. Acetone is the most aggressive stripper/solvent for this job that I’m comfortable working with. It is also what’s called a “transdermal conductor”…it can transfer other chemicals that are dissolved in it right through your skin…so USE THOSE GLOVES. The big roll of paper towels shouldn’t require any explanation. The other white stuff is a generous piece of Stits PolyFiber left over from my Stinson SR-9 project.

When you have the fabric as clean as you can get it and any slip-ups fixed, you’ve got it made. You can now brush on a new substrate of several coats of clear nitrate dope (Sig sells it as the recommended base for their Koverall fabric) OR you could go the Stits Process (I would) and begin building up your finish base with PolyBrush exactly per their manual. Check out www.stits.com for all the info you’ll ever need on that score. Stits finishing products are fuelproof, if that matters to you, and will work OK on Koverall or some other polyester fabric if it turns out that’s what you’ve got on the airplane, although none of their warranties would apply in that case. If you get into the job and have more questions, let me know and please share a photo or two of the restored airplane when you get it the way you want it.