You'll have to follow the tech sheets for whatever products you plan to use. For my last two paint jobs, I have used the products from www.tcpglobal.com and www.autocolorlibrary.com I have used their "restoration shop" brand primers (epoxy primer as well as 2k urethane primer) and their brand paint in both their pre-packaged color as well a factory color.

My method was:

-Sand body down with 320 to scuff it up good, burn off clear coat, etc.
-Do the body work, metal repairs, welding, filler(shape with 220, finish with 320), etc
-Epoxy primer
-2k urethane primer
-Block sand 400 - the urethane primer sands nice and easy plus super smooth.
-Paint