sanding old paint

hey guys, i wnt out today and got a new 6" DA sander. 100 pack of 80 grit paper and a 33 gal compressor set up. 300 dollars home depot, good deal. came with alot of air tools. anyways, i hooked up the sander and it ate through the paint big time. should i used a finer grit paper. like 150. i dont want to get it down the metal, just in some spot, if i continue, i will mess everything up with the 80 grit. where the paint is good, i just want to scuff the surface so primer to stick right? please help. oh, and paint will cost 560 plus 200 for primer, does that seem high to you? its good paint, i beleive PPG. what you think