[Mike 85]

Any epoxy needs 24 hr to sand without clogging the paper really. I use to use Nason and it sands "ok" up to a week after spraying,After a month or more,it becomes concrete and 80 does well to even cut it.Last job I did was a dragster with 17 seperate body panels and I shot 2 coats and blocked with 80,did minor filler work on dings and some E-coat FiberTech for the nose glass repair and 1 coat of epoxy to seal it and Nason 2K primer to finish blocking it for paint.
Guide coat and block with 80 initally to SEE what you have and do your bodywork and filler,block it out and spray another 2 coats and reguide and block with 180.You can either finish sand with 4-600 wet depending on what kind of paint your using if it's "straight" or spray your 2K primer and finish it out. I get confused with this whole "prime,surfacer" deal too.
You basically have 2 kinds of 2K primer. Regular 2K primer and High Build 2K.
It's a mix bag with primers and you basically just have to read the tech sheet on each product as to what exactly all it can do.
Some are just primer.Some are high build,Some can be reduced and used as a sealer. Insanity at it's best and confusing as anything in the paint world.
Main reason I stick with epoxy anymore. I have 1 product and can do about anything I like with it from start to finish.