The answer depends on the condition of the paint that is already there. If it is in fairly good shape, scrape off any loose paint and rough sand the old paint. If there is a lot of flaking that will leave the surface really rough, fill the voids with exterior spackle, then sand smooth. Use about 100 grit paper for either of these. You could strip the old paint but if the windows are in pretty sad shape, it may be cheaper in time or money to replace them. And baked on paint over aluminum or vinyl will let you enjoy your time at the beach rather than working.