Dried Primer Not Sandable?

Hi, I'm in the process of preparing my first fiberglass sub enclosure for paint, and I used a couple coats of Bondo autobody filler and sanded them down, and then I sprayed two coats of high build primer onto the enclosure and let it dry overnight. The primer is now dry and really soft, and can't really be sanded. When I use 80 grit sandpaper, it carves large grooves in the enclosure. When I use 220 or 320, it causes tons of little bumps to form in the area that I'm sanding. Am I just retarded, or is there a trick to sanding primer that I'm unaware of? Also, I think I may have put on the primer before the enclosure was ready for it. There were still some holes that I figured high build primer would fix, but they're still there and the primer doesn't seem to have filled anything at all. Do I need to somehow remove all of the primer and put on more autobody filler and get everything very smooth before I primer it again?

Sand all the primer off. When you think the box is ready to prime go to Wal-mart or wherever and get a car paint primer that is wetsandable. Use wetsand sandpaper after you apply the primer. about a 320 grit then 400 will work good.dream it, build it, fiberglass it

2 things....did you mix the poly primer extremely well before you poured it into the mixing cup? and second, did you mix in enough of the MEKP to harden it. if not it will take forever for it to harden fully. what brand and kind of primer did you use? Trevor

get a few sheets of 240 and 400 wet sand paper, a bucket of hot saopy water and you wont have a problem with the primer building up on your paper, start do most of your work with the 240 then sand the scratches out with the 400 it take a bit more time but its a lot less mess and its harder to mess up