I assembled SCCA V-8 & V-6 engines for a shop several years. We got away from painting for the same reasons listed below. The real way to speed oil return and get rid of any loose casting is to use a die grinder with carbide and sanding rolls.
The first return season of freshening engines that were painted had paint starting to flake from jet washing or hot tanking the blocks. I don't about you but I don't want any "electric motor" paint in my or customers engine bearings.
Look up Y112 photo gallery for 427 block prep picture. No overspray, just 4 hours max work for big block