Do you want to fill holes which are no longer used? If they are in solid glass, then you want to make a filler up of polyester risin, and either mill fibers thickened with Cabosil or a medium density filler. I like to drill the hole clean, no matter what size, and then clean with acetone. After filling the hole (and if it is bigger than 1/4", I would stuff in some random mat fibers--if more than 1/2", then I would feather the edges on both sides (if possible) and lay in glass mat and cloth in concentric circles.--then fair.
Finally gel coat (Spectrum color has gel coat patch kits which match and are easy to use).
The obvious "easy" way is to use "Marine Tex"--an epoxy. There are some disadvantages to using Marine Tex, but I do use it occasionally.
It is difficult to gel coat over, and for a truely good repair, a good glass shop will remove all of the Marine Tex. Epoxies are also very sensitive to UV degrigation, so should be painted or coated over. MarineTex will fade or yellow.
For re-laminating and building I love West Systems epoxy--and it gives a better bond than polyester. But the difficulty gel coating it, leaves it a bit behind (a tie coat is best if you want to gel coat--such as Duratec polyester surfacing primer)
If you have a more specific question, I would be happy to answer it.
