[LS short box]

I'm not a body guy but I play one for 8 hours a week at a buddies body shop. LOL.
I'm the king of block sanding on some of his resto projects.
My buddy won't spray any single stage. Or won't spray any paint on poorly prepared sheet metal.
Bare metal and properly prepared original paint gets a coat of epoxy primer. Then a skim coat of filler. Then a coat "poly" primer. Both sanded down to almost nothing to get panels straight. Then a couple of coats of sanding primer.
Then three layers of base coat. Any bad stuff that shows up in the base coat is sanded out and resprayed. Then 3-4 coats of clear. Cut and buff to follow.
I've done a number of BC/CC hot projects over the years. Never had the clear coat fail.
Pic of my 68 in bare metal. Pic of it painted.