What is the procedure for prepping a boat for fresh gelcoat? Do you have to sand down to the fiberglass or can it be sprayed over the top?

Also, how much would you typically need to spray the bottom hull of a 16' boat?

I'm currently working on restoring my 1984 16' Skeeter and there are some chips in the existing gelcoat and one area of moderate fiberglass repair needed at the bow. The color scheme is a brown with flake on the cap and a cream/brown/tan sort of color on the hull. I could color match the brown and the flake on the cap, but I'm not so certain I could match the hull color. The brown flake has a few scratches, but nothing major so I will probably leave it be and just buff it out as well as I can. Since there are so many chips and I have a significant repair to do on the bottom, I'm thinking I may as well shoot the entire bottom hull.