I use dope.....
Midwest(formerly Pactra) butyrate dope. Straight on, about 3 coats clear firewall area, inside tank compartment, around nose area of plane, and any other area that might be suspect to seepage, depending on planned exhaust exit. Then usually paint black on firewall, just to leave that finished looked, which it does very well.
It's cheap, works well, very easy to use, the lightest substance available and drys in 30 min or less. Doesn't get easier then that, been doin it like that for over 25 yrs, I can tell you the dope lasts alot longer then planes.