For thinning, consult the epoxy manufacturer's instructions for thinning. Recommendations vary by brand. I run/race nitro wood hull boats/65% nitro, Bob Smith Ind. finish cure epoxy thinned with 97% isopropyl alcohol per their instructions has held up remarkably well in the engine & fuel tank bays. Automotive 2 part/catalyzed clear holds up very well, also & is lighter than epoxy. I have never had any luck with any single stage product, if you can buy it in the paint dept. at a big box home improvement store, it ain't gonna work for long or very well. The only exception I have found in single stage products is Lustrecoat spray clear. But you need to let it set up for a while, a month minimum in my experience with higher nitro percentages. Claims to be fuel proof to 25%, but I wouldn't spray it today & fly Sat. With an extended cure time, it holds up OK for me. I will only use it on the outside of my models, mainly due to ease of use. It is lacquer based, & goes well over the Tamiya spray bombs. Quick & easy for a down & dirty job when I need to. Like any finishing, base, mid & top coats need to be compatible & manufacturer's instructions need to be followed for best results.