I mix 1 part microballons to 3 parts BONDO (yes Bondo). Mix the microballons THOROUGHLY into the Bondo BEFORE adding the hardener. The end result is relatively light. It has substantially more strength than a light weight filler (less edge chipping and cracking ) and is fairly easy to sand. It is going to sand "harder" than a light weight filler, but no where near what straight Bondo would be like.

If you decide to go this route (or use the SIG Epoxy-Lite ) have some 91% rubbing alcohol handy and smoothe the surface of the Bondo while it is still wet. The alcohol will allow you to shape the fillet easilyand the Bondo will not stick to whatever you are using to shape it. Make sure you let the Bondo dry overnite before doing any sanding. The Bondo may APPEAR cured, but it isn't. It will take several hours for the alcohol to fully evaporate from the surface (and the little bit it will soak into the Bondo ).

For your engine compartment and fuel tank compartment - thin the epoxy with either acetone OR DENATURED alcohol. Thin about 10% by volume. Regular rubbing alcohol contains water and epoxy and water do not mix.

Hope this helps.