First question is...do you see any peeling, cracking, or blistering? If the paint is failing, and you paint over it, the new paint will eventually fail too.

Second question...How many paint jobs are already on it, and what color are you painting it? If there is more than one, maybe two, you should strip it, especially if you are going gloss black, or something like a candy or pearl color.

Third question...are there places where the old paint job has weathered through the current coat of paint? Are there very many scratches, chips or "dings"? If so, you should feather and repair them, and bury it with 2-3 coats of catalyzed primer.

To answer one of your questions, yes you can paint over sealer, but it has to be done right away. Read the directions before you start.