Richard,

I filled any visible imperfections before I glassed the fuselage. I used two coats of clear to lay on the glass, sandedwith 180 grit followed by two sprayed coats of clear. Now I will block the clear and if it sands out clean (no imperfections) I will paint without having to use primer. If there are imperfections after blocking, I will apply two coats of primer and then reblock everything. Remember the goal is to get as smooth and straight a surface as possible without too much weight build up. In this case "less is best". That is why I won't be using primer unless I absolutely need to.