Can't answer your question specifically as I have no experience with PA on urethane paints, but I assume you want to paint your topsides and wonder why you need to remove it all. Sanding it would be one way to do it, but I doubt you have to remove it all, but rather just sand it smooth and frosty (to get a good tooth for the next coating). After cleaning, then you would want to apply the primer for the topside paint you intend to use. For one, it helps give you a better adhesion of your final paint and for two, it offers up a barrier for your color coat should it be a more aggressive paint than what was on there before. For example, Awlgrip's linear polyurethane paints are much more aggressive than most other paint types and might lift the undercoating if you didn't apply epoxy 545 between the two.
