I guess I don't understand why other people have trouble removing old paint

A good primer surfacer will fill an 80 grit scratch in two coats. Scratch the metal with anything more and it will take more coats of some very expensive paint to fill the scratch. That being said I've never had any problems with Mirka or Norton 80 grit clogging quickly. Maybe it's the type of sandpaper being used?

Guess I've always looked at in the overall process: the smoother the sheetmetal, the smoother the undercoats, the shinier the paint will be. The paint work is only as good as the prep work. And if you miss something, like an improperly filled 36 grit scratch, in a paint job that's deep enough to read a newspaper in from 5 feet away, it'll stick out like a crater on the moon.

There's also the productivity factor to take into account at times, too. Back when Chevy was warrantying a paint job due to delamination, I could bring in a full size truck first thing in the morning, strip it of the door handles, emblems, mouldings, grille, etc. Hit it with chemical stripper to get the paint off, and then go to town on the primer with 180 grit and have it to the paint shop by noon. Nothing courser than 180 as the painters didn't want to have to block primer in before paint. They'd paint it, bake it, back to me for assembly, and it'd be out the door before quitting time. Looking better and outlasting anything the factory ever did.