Hi
This is really one for Mr Gelmonkey, as he is the expert on this stuff, however I have a little experience,
I am assuming you will be using a basic colour coat top coat, not a colour coat and clear coat system
first thing is to get it all sanded down much as you would any surface you wish to paint, take it down to about 180 grit finish, don't go too fine you need a good key, however the surface needs to be even. do the classic sand at 22 degrees motion you would for any body work (first stroke at 11 degrees off to one side to the direction of the pannel front to rear of the car, second 11 degrees the other side, so you get a sort of herring-bone effect in the sanding marks. Use a guide coat to help find any low spots and use a long sanding block, at least 8" long more if it is a long flat panel. Give the panel time do dry fully and use an electric fan heater to get it really free of moisture, when dry (did I mention you need to get the panels dry
) wipe off with thinners to degrease it. Put down a high build primer/ filler with the spray gun using 2/3 wet edge overlap, again sand when dry as before but this time take it to 240 grade, again high build filler/ primer again sand, this time to 600 grit put down a nice thin coat of colour, 1/3 overlap on a wet edge, when tack dry put down a second coat the same way, leave to dry thoroughly (couple of days) block sand starting at 600 grit, then 800 then 1200 grit, but very lightly on the 600 and 800. Let it dry, give it 3 more coats of colour as before 1/3 overlap, leave it to dry, at least 72 hours preferably a week, then colour sand using 2000 grit, then polish and hay presto nice shiney panel.
That is a very old fassioned way of doing it and assumes cellulose type paints, or modern equivilent water based acrilic colour top coat systems. I have never used urathanes, cyanoacrolate 2 packs or epoxy systems and I have not used colour plus clearcoat systems (the latter are meant to be much less work and give very good results.
Basically it is like repainting any car, all in the prep, and provided the gell coat is stable and a good key surface is used then you need nothing special.
Best regards
Mike
PS there are loads of how to articles on the carcraft archives.