From painting our boats I've found the original gel coat to be peppered with pinholes, I suspect aftermarket glass parts will be the same. A spray polyester surfacing agent like DuraTec works well for cleaning this up. DuraTec is basically polyester resin and microlite, a sprayable Bondo which you catalyze with MEP (just like Bondo and conventional polyester resins). You get 10-15 minutes pot life at 70F. Add acetone to thin it out. As opposed to using Bondo the DuraTec applies evenly, fills the pinholes, and sands like talc after curing for 30 minutes.

If you brush on a layer of straight polyester resin, use finishing resin which contains wax, or spray the surface with a curing film like PVA. If you uses laminating resin by itself the surface will remain tacky even when dry, it will gum up the sandpaper very quickly. Clean the wax or PVA off before sanding.

Be careful with the new parts before painting, gel coat is quite porous. It will absorb oils through handling which will cause problems when you start to paint. Even washing with acetone just gets the stuff on the surface. Best to avoid the problem by protecting as much of the part during mockups with a low adhesion protective paper like ProTex or Mask-Off.

Before doing anything with a new fibreglass part -- clean it well. The part will retain trace amounts of the mold release agent from manufacturing. Some of these contain silicones which are death to paint (unless you like the look of fish-eyes). If you start to sand before cleaning you transfer the silicones to the sandpaper then grind it into the gel coat.

As I stated my experiences have been working with fibreglass boats, but should be the same with fibreglass car parts.

Regards, Mark