Re: Why paint instead of gelcoat?
Seems to be a little confusion here about resins (which includes gelcoat).
Most gelcoat you see on the shelves of your local marine store include wax, which will rise to the surface after application to create an air barrier that will allow the resin to properly cure. You can get -- and I prefer to use -- gelcoat without wax; the surface of this will not fully cure and makes a chemical bond with the next layer of gelcoat possible. You can then air seal your final coat of gel with PVA or with a film (acetate works well for relatively flat repairs.
Likewise, polyester resins come in two flavors: standard resin (some call it "boatyard resin") which will fully cure; and laminating resin, that even after curing has a "non-cured" surface that will allow a chemical bond with successive applications of resin and eliminates the need to sand/prep between layers.