Generally you don't sand epoxy primers. On cast parts you just prime and topcoat. On sheetmetal parts where you are trying to smooth and level the primer you would apply a filler primer over the epoxy primer and sand that. Be sure you get a data sheet on any paint you use and follow the directions, especially the recoat window on the epoxy primer. Epoxy primers have to be topcoated withing a certain timeframe or have to be sanded or re-primed. Someone else more familiar with PPG primer could tell you which would be the best surfacing primer to use with the mp 170. The part of the scan compatable surfaces may give you some idea of ppg primers you could use.