hmmmm, best way i could think of would be first scuff it up with some 80 grit sand papaer or scotch brite...then put some stuff called bulldog primer on it then put some 2K/highbuild primer on, i'd do a couple coats on then sand it down...then paint your color on...that would keep it the straightest...if you just try to sand it as it is then it might look wavy or warped...

just depends how much time and money you want to put into/get out of it