Body Work Tutorial

Ok here is what im working with. You can see from the photos the top coat is chipped and flaking badly. But once I started sanding with the 50 grit I noticed the scale was leaving but what was underneith was actually pretty good shape. So instead of taking to bare metal id like to take it down far enough to get rid of the scale. I bought some disc awhile back I have 50 of each 50grit 180grit 220grit and 320 grit. My epoxy says 220 as min for primer. So what between the 50 and 180 should I use? My plan is 2 coats of epoxy followed by 2-3 coats of 2k high build, block with 220 then 1 more coat of 2k wet sand at 400 then apply sealer then top coat.

Now above is tenative I used the 50 grit right or wrong its already done. If something above is wrong let me know. Im doing this for a first time and I do listen well and its going pretty good. I do need some filler work mainly where I installed quarters and a few other patch areas. Not much for dents and dings really.