Steve:

What 'synthetic' material are you proposing to put a two-pack pain
over?

Since this is a boat-building forum, I take it that you've primed a
polyester resin, gel coat surface and are now wanting to paint it with
a two-part polyurethane paint. I also assume this is a 'topside'
(above the waterline) application?

All two-part paints (that I'm aware of) require a specific 'epoxy'
(i.e. two-part) primer or under-coater. You cannot apply two-part
topside paint over a primer not designed for two-part polyurethanes.
See : www.blakespaints.com for additional information from this UK
paint manufacturer.

Yes, you can re-apply as many coats of two-part polyurethane 'on top
of' each other as you want, (generaly) sanding between coats.

Mike Worrall
Los Angeles