First of all Welcome to the forum. Now without seeing a picture of the ares in question "which would be nice if you could post one" There is actually a few different ways you could go here but this is my recomendation. Start by sanding the whole thing with about 150 to 240 grit paper, the areas that are exposed fiberglass can either be filled with a short strand fiberglass filler "it comes in a quart can". This basically is like applying body filler but it has fiberglass strands in the filler. Or you could just use a polyester body filler as this will also stick to the bare fiberglass as long as it is clean, you may then sand that with 150 to 240 grit and proceed to primering. Good luck and let us know if you have more questions.
