These new products were designed for using finer grit papers , not the old route of 40 grit then 80 grit then prime . Some are even designed over properly preped paint as by BMW and Mercedes recomendation .
They are ment for 80 grit to start with and finished off with 120- 220 grit , then prime , this speeds up the paint process in a shop ( minimal scratches = less material= faster dry times ) , as the paint always took the longest time of the repair side of yesteryear , not so today .
I also have been using the UV primers , 2 minutes on the UV light , and wipe with a special chemical rag , sand and paint . Bad side is its for small repairs 9"X9 " area Max .