No the fuel will not eat the foam. I have not had the P-39 but I have flown the 1/2a Piper Cub, Spit fire, P-40, Cessna 182. I covered all in an iron on film. I used Flite coat, Color Tex for the cub, and econocote. They flew alright. I did like the fact that they will take a hit and just bounce. The foam is pretty tuff sa little bumps do not hurt them.

Cover all gaps in the control surfaces. I installed balsa block in the area of a hinge to spread out the loads. The P-39 would look good with retracts.

Dru.