Sorry for all these posts on stuff I should have known 30 years ago.

Preface:  I've painted a lot of models in my life.  For years, I used almost any brand of dope (SIG, Stits, Brodak, Aerogloss, TopFlite Lustrekote (whatever that was).  Then I went to auto paint, both two stage and enamel with hardener  (I can't use either of those right now, but life is all about learning). 

And I always worry about paint being fuel proof, especially with the biplane I'm now building.

But looking at the models on the plane tree last night, something occurred to me:  I have never had paint actually soften to the point of smearing and coming off.  Not once.  And I'm not particularly diligent about wiping planes down.

What I HAVE had happen, especially with some of my fleet that is now more than 20 years old, is the finish lifting off in places, or having bubbles coming up in places particularly around the cowl or forward joints in the fuselage sheeting (never on the wings).  Just thinking about it and my best guess is that fuel got underneath the paint or in the wood where the initial coats were applied to the balsa.  I will also say, that with the exception of maybe putting a coat of butyrate in the engine bay on occasion or smearing a coat of CA inside the engine compartment on one or maybe two of them, I've never paid particular attention to preventing fuel from soaking into the wood, and I know some of that stuff is going to get past the firewall into the fuselage anyway. 

So, I'm thinking that a really good fuel-proof paint or clear coat isn't really going to solve my underlying long-term problem. 

On my current build, I'm using (used, I'm starting to sand/primer now) Minwax Polycrylic, which probably isn't fuel proof.  So now, I worry about adhesion to wood.  The engine bay is still open and I can take some steps to fuel proof it.  And I probably need to address the open edges of the balsa when I cut/drill the holes for the needle valve and fuel lines.

Am I crazy about thinking the long-term adhesion is as much, or more, of a problem that the paint itself ??  Well, that was an unfair question, since I'm pretty sure I am crazy.

What steps do you guys follow for protection from the finish lifiting/bubbling ??  I would assume the best way is from step one when applying the first coat of whatever to the raw wood.  But it's too late for that.

https://www.klasskote.com/
The hidden cost of owning a model bipe is the clean up...!!
The Klasskote primer sands like a dream and I assume is fuel proof as well.
This stuff is extremely heavy...so I might limit it to the cabane struts and the engine area.

  Once properly sanded, it is significantly lighter that almost all other primers and sealers. I sprayed about 3.5 ounces on my entire airplane, sanded it back down to 2 ounces, and there is no other filler or other sealer on the airplane. It filled the carbon grain with the exception of some pinholes, which were not fixed due to time constraints, but would have required unmeasurable weight of filler to correct.   That is *very light" and the total finish is only about 7.5-8 ounces, which about 4-5 ounces less than an equivalent and much less durable dope finish.

       Brett

Brett what types of paint have you shot above the KK Primer and how much weight is usually in your budget for the size of plane you compete with...?

   I used Klass-Kote, K&B Superpoxy, and HobbyPoxy over KK Primer. Jim Aron used automotive colors and dope (not sure what kind) over it.

    My finishes have come out around 6.5-8 ounces for my 680 square inch airplane.

     Brett

I've only used the 2K automotive urethane [Summit Racing brand] on cars and so  I have some idea how fast you can "blow through" a 6 oz batch.
It's tough paint..which also means it takes quite a bit of elbow grease to correct any flaws the following day.
I'm not sure how long it needs to cure before it is truly fuel proof.

    To be entirely clear, Jim Aron uses either automotive colors or custom dope colors over Klass-Kote primer. Jim, the late PTG, and I use 3-part polyurethane clear over any of these. I would never, ever, use dope clear for the top coat, it is far too fragile. It adds about 3/4-1.5 ounces to do the entire airplane. This particular clear is very workable for about 2 weeks, and still can be usefully corrected almost indefinitely. It is effectively fuel-proof after about 12 hours, but clearly gets thinner over about a week.

    David Fitzgerald used clear dope on the WC airplane, but put 3-part urethane on selected surfaces for fuel-proofing.

     As far as weight goes, starting from doped carbon matt, I figure about 2.5-3 ounces for primer, 2-2.5 ounces for the white, 1/2 ounce for the red, and 1.5 ounces for the clear. Everything else is negligible. Next time, presuming there is one, I plan on using a blocking coat of very light blue, I figure that is another ounce.

    My primary finish flaws are pinholes from the carbon matt sizing. To fix this would be trivial, get it up to sanded primer, use Bondo with a soft plastic applicator to fill the pinholes where necessary, sand flat, paint. The reason I haven't done that on the two airplanes I have had with pinhole issues is because I have to paint at other people's houses, which is an extreme imposition - not to mention it is 2.5-3 hours through Bay Area traffic. I haven't been willing to burden my friends that way.

     Brett

Brett..thanks for the details..!
Pin holes are such a nightmare because in some cases you are just a couple of sanding swipes away from finding the next layer of bubbles that are lurking  below.
BTW...I grew up in San Anselmo and ever since I can remember.......... Bay Area Traffic has been horrible.
Back in the day the roads would be decent by about 7PM  so that we could  make the "reverse commute" to catch a night game at Candlestick.
Of course back then the games started at 8PM and they were usually over by 10:30. Not too bad.
We even had a relative on the team to root for, for a few years...his name is Jack Hiatt. Hit his first MLB HR off of Koufax...imagine that.

   In this case, I think the pit goes all the way down to the wood. You could fix them before primer, but it would be almost impossible to see them so you would have to bondo the entire surface, and you would never get enough of it off. After priming, they stick out like a sore thumb, you can locally fill them without leaving any where you don't need it, so the weight buildup is zero. You need something like a putty because any liquid would just flow away from the hole.

  At any rate, this is not a huge problem, it probably keeps me out of the 19 row, but routinely gets 17-18, which is good enough to not knock me out on Top 20 day. All the usual suspects are 17-19 most years.

       Brett

I would like to recommend this stuff .....https://www.amazon.com/evercoat-spot-lite/s?k=evercoat+spot+lite
but the price must be twice what I paid for it just 3 short years ago.