I have guidecoated and am ready to base coat. I have used a Polyurathene primer and I would like to know if I can paint over this or do I have to use a sealer on top of the Polyprimer?

If you have any metal showing you need to hit that with a little sealer first. I don't like using sealers over the whole car, it is just one more thing to give problems. Also the sealer will not be as flat as the wet sanded surface.