you can paint straight over 2pac as long as rubbed sufficiently, if painting metalics rub with 800wet if painting solids 400 on a sander will be sufficient. the only reason i can see needing to use 2 pac primer over 2pac is over repairs and whatnot as at work we use 2 pac and just prime repairs. if blending panels on metalics we rub with 1500wet then use scuff it or a 100disc by hand just to ensure you havent left scratches from rubbing incorrectly, should always use palm not fingertips as fingertips will scratch it up badly and colours with metalics will show it right up!
at work the painters have respirators with filters on them and a parachute like material pants and jacket 1 to stop them gettin shit and the paint and i gues to stop the paint gettin into their skin but when priming we do it out in the open and i don't have a respirator so hello to an early death for me!
etch primer alot like 1k primer, which is acrylic can be painted over with 2pac but isn't recommended we use it a bit in a can at work mostly to prime over small rub throughs in repairs but rub it back before painting over it also.
we use wax and grease remover, sometimes metho, metho more for on new plastic parts as prepsol (wax and grease remover) is too harsh on the unpainted plastic!