I have parts or entire cars sitting inside in sanded 2k for years. Poly primer is a pain in the ass. I only use it if you need excessive mil build to straighten. It kicks faster, sometimes requires extra large tip size, more difficult to clean up, more expensive, harder to sand than 2k, and I hate those stupid hardener tubes. If you're putting epoxy down first there's no issue using a 2k urethane, sanding, and waiting till spring. I can't imagine trying to sand poly with 400. It will load up the sandpaper so fast. That is why any time I'm finished with a blocking step, I spray a final coat of 2k to fill in scratches, and wet sand the 2k with 400.
Otherwise you're left with having to sand a lot more to cancel out the 220 scratches with 400. You think you're done and you still see 220 scratches. Once my 2k is wet sanded with 400, I shoot 25% reduced epoxy to seal and it's ready for paint.
Handy trick originally learned from Barry years ago.