I'm no bodyman, but from what I understand, you can block epoxy, but it is very difficult. Some epoxy primers sand easier than others...and there are people out there that block epoxy, but is is a lot more work.
If I were you , I would direct these questions to: www.autobodystore.com
Len has a great forum on there that is full of experienced bodyguys that are more than happy to answers questions such as yours. I enjoy just going through the forum and reading...it is amazing what you will learn. Hope this helps.
Good advce by Gambitt, I have learned alot at the autobody store message board, lots of pro's hang out there.
Here is what you will find out if you ask enough people about epoxy primer. First off it is the best choice for bare metal. Next read the tech sheet on your brand of primer, most epoxy's stay open for up to a week, and after they cure up they need to be scuffed up before adding more primer. If you recoat within the recoat window you do not need to sand, maybe just hit the spots that trapped any debris.
Epoxy is not ment to sand , and when it is green will really gum up your paper.
I think most pro's will agree, that after your epoxy coat, and within your recoat window, come right behind the epoxy coat with a few coats of 2K sanding primer and go from there blocking and spraying more 2k primer as needed. And as the last step before the color, seal with one slightly over reduced coat of epoxy primer or a dedicated sealer.
Good luck Mike