What brand of paint is on there now?
I have alway's heard that if you use the same brand of paint and the condition of the old paint is okay, then you can paint over it. As long as you clean the surface of the old paint. If you decide to strip/sand the old paint, I recomend a product at Sherwin-Williams called "PEEL AWAWY 1". It did great on stripping my old bottom paint off, then using a presure washer to get off the remaining residue. Hopes this helps. If you need more info you can e-mail me at budwiser1@fuse.net