I was in the same hole but all that sanding and re-sanding just means that your panels are not blocked. It will all pay off in the end. What I was runing across this I would start with 220 finish with 400 before adding another coat if the bottom coat was showing again. after a few times I shot a heavy coat of G5 feather fill that fills more than Urethane primer. that made it the final coat of primer I had to block. I then applied a coat of Urethane and wetsanded to use as the sealer.
My first car Im very very happy with how straight my panels came out