I've had a lot of boats (used) Gelcoat is a wonderful material, very forgiving. Have a hose in one hand and a piece of 400grit sandpaper in the other. Wet sand the bottom and sides, this does take awhile, after all discolor is gone switch to 1000grit and do the same thing. I use 3m finesse it, wipe it on and buff to a brand new shine. I usually throw on some wax after. This works best on white gelcoat because it is so forgiving in showing scratches. If it is white you can do all this in just a couple hours. If it is a cream color or any other darker color, after the 1000grit goto 2000, then buff. Anyway just my experience. I have taken boats that were chaulked white, and the original color is yellow...brought it back to a sweet original wet looking yellow again, by doing this.