you can wet-sand "sandable" primer, as long as it is sealed with something on top (clear, color, sealer, etc)
when I bought this 1967 Dodge dart, it had no hood, fiberglass doors with horrible lexan windows, and was in a state of "rattle-can hell". The trunk was baby blue, and I did the following:
bought a white hood, snagged hood springs from bone yard from a 76 Valiant, bought steel doors with glass and wing-windows from a 76 Duster...they were gold in color. I then used a DA(dual action grinder) and 1 roll of stick-on 180 grit PADS, followed by 400 grit. block-sanded the hard edges of the body lines, rattle-canned the whole car using newspaper as masking, wet sanded the primer by hand and rattle-canned it with sealer/clear.
The car was called "the Roach Dart" when I got it, and it had a 383 with Offenhauser intake and edelbrock carb. barely ran. I put a Torquer 383 intake and Holly 650 vac. secondary on it and it ran like a top. It was a track-only car but if I had kept it It would have been on the street by now, and worth about $10k by today's prices.
I sold it to buy our S10 drag truck. This Dart sold on craigslist in 8 minutes....for real.
I posted it, got up to take a break, came back and had a text waiting for me. the car was gone in less than 2 hours....shoulda asked more!