I had some scratches on my bumper and I sanded it down to black, then taped and did two light coats of primer. The primer is bumpy and seems to stick out from the regular paint. So I sanded with 400 wet sandpaper to try to blend it more with the stock paint and it all came off. What can I do? It seems like one coat of primer is thicker than factory primer, paint, and clear coat combined.