Second Round of Blocking

I am about ready for my second round of high build. The first round was done with 180 grit.

I am now going to move up to 320 grit. I know I want to move up to 400 before the I lay the sealer coat, but my question is do I have to lay down another coat of high build and then block with 400 after the 320? Or can I just go over the 320 with 400?