I use clear polyurethane paint for the tank compartment AND the radio compartment. I just pour some in and slosh it around. I use a disposable brush to get at areas it didn't slosh to. When it's all good and wet I tip the plane forward and let the excess drain from holes in the firewall.

If there are any puddle in corners I just soak them up with a paper towel. It takes just a few minutes and gives good coverage. Usually I do it twice.