Last time I painted underneath on of my buses was my late bay, back in 2007.
From memory I think I repaired the neccassary areas, seam sealered round the patches/ panels primed those bits, scraped off all the old underseal , cleaned the oginal paint up , keyed it a bit then blasted everything with over paintable stone chip then 2 k paint.
I still waxoyl'd all the nooks & crannys ( hard to reach) bits too though!!
It's a bugger to keep clean under there mind!!
Hence my last 2 buses just got black Schultz'd :lol: