I have a skin on frame kayak currently with polyurethane coating sealing a nylon skin.
I need to apply an additional coat to fill the weave but have no more polyurethane.
Will EF80 adhere to the polyurethane?

You would need to key the PU for the EF80 to get a mechanical grip.  The bond between the two materials will not be very strong so it is possible the EF80 may peel eventually off the surface.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant