Basic prep if its in gel coat, just scuff it up with a red scotch brite pad if its "almost paint ready". Now that term can mean a lot of things to different people, so keep that in mind.
Here is what I do to prep fiberglass parts.
1 - clean the surface really good with a degreaser/wax remover. You can get this at any auto parts store, and even walmart. Also, wash your hands and try not to touch a bunch of grease or something. Cleanlyness should be at the top of the list at all times. A good paint job all comes from a clean surface.
2 - scuff with a red scotch brite pad like mentioned above. Do this until the surface is dull all over,.. no shiney parts at all. Corners and edges are important, as thats where the paint will peel first.
3 - As Crazy Beavers said, use a good quality primer. I prefer epoxy based primers, more specific I only use Martin Senour available at any napa stores (that comes from my training, although I have used other brands with success). I use the high build primer because it fills like bondo, and sands easily. DO NOT use cheap rattle can primers, you will regret this. It may be the same paint as whats in the can, but I have never used a rattle can primer thats worth a damn when it comes to sanding and filling.
4 - Again, as crazy beavers said spray a guide coat on. This will show all high and low spots, and any imperfections. Fiberglass is notorious for having small pin holes, ones you normally can't see until something like a guide coat points them out. Sand the part until there is no more guide coat.
Now, step 4 can either be done dry sanding with 400 or 600, or wet sanded with 400 up to 800 depending on how well the surface looks. I normally dry sand a thick coat of primer in step 4 with 220 grit. Then I follow it back up with another thick coat of primer and wet sand it out with 600 then spray my base coat. There are a lot of different methods, and just about every painter you ask will have a different method and pattern to which he follows. Every way gets the job done, so none of them is wrong. Some guys shoot 4-5 coats of primer and block sands each one until they are happy,.. but thats mainly on high end show only cars for high profile clients (think Chip Foose).
Once your happy with your part in primer, your ready for sealer or paint. If your primer job has 100% coverage, most times sealer is not needed. Sealer is mainly used when blending touch up jobs and repairs into complete panels that are already painted.
Youtube has a lot of how to videos on this subject. I suggest finding some that you feel are informative, and asking on here for us to watch that video. I would feel safer with seeing the same video your seeing, and making sure the information is correct. If the video is jimbob in his backyard painting parts on top of cardboard,.. DON'T LISTEN TO HIM!!! Try to stick to videos that are filmed at a Vocational school, or something of that nature.
