I thought I answered all of this when you first asked the other day? See link below.
https://stunthanger.com/smf/paint-and-finishing/finishing-foam-wing/ You can manage to color foam but not make it fuel proof. Lots of different tissues and wrapping paper out there to layer on and cover foam just to cover it, and then you can paint it with almost anything. On a small 1/2A combat model if you keep the glue and such to a minimum you won't build up much weight. Your problem will be with spilling fuel on it. The exhaust won't hurt the finish unless it's so rich that unburned fuel comes out in the exhaust. If you get a thin, even layer of glue and paper on, then the popular urethanes that some guys use will work also. But you have to put something on the foam first.
So even after asking three times, the answer is still no. There is nothing you can spray on foam that will cover the foam and be fuel proof..
Type at you later,
Dan Mcentee