I sometimes spray Varathane Satin Finish over my ink lines, etc. on the finished, painted model, and it holds up well to normal fuel exposure. I believe it would also be fuel proof inside the tank compartment, etc.

This may be poor advice, but after some 28 years and 113 RC models built, I have quit fuel proofing the tank compartment, since I have only had one tank spring a slight leak in that time. But with the reputation of some of the tanks that come with today's ARFs, you may want to continue to fuel proof the inside.

Clair