RE: Fiberglass Cowl/Pin *****s
A lot of people use liquid dishwashing soap which can be bad news. Use a real degreaser such as Tri-Sodium Phosphate (TSP) to degrease before painting. Dishwashing soap (depending on the brand) will cause fisheyes.
Also, if you're having problems with the filler sinking in then you're probably not giving it enough time to dry before sanding. A lot of folks are impatient and sand it as soon as it feels dry. It can take literally days or weeks for this stuff to dry so if you sand it 20 minutes after applying it then often it will shrink more as time passes and leave indents in your finish.
The number 1 most important thing to a finish is sandpaper. The second most important thing is letting each application of whatever dry as long as possible.