DP epoxy is designed for bare metal and acts to seal the metal from oxygen and keep it from rusting. It's non-pourous and not designed to be sanded. There is an ongoing debate about putting filler on bare metal or on top of the DP. PPG latest instruction is to put filler on top of the DP. DP doesn't sand well at all, making a bit tough to feather in the filler, but it can be done. You can use a sandable primer or high-build primer on top of the filler and DP to get the car ready to block. You can then lay DP sealer on top of all that, but you would wait until the day you are going to shoot color. Once DP sets up, you have to scuff prior to laying down the next layer. The original layers of DP will be entirely scuff before you go to the high-build!
I didn't know that they offered a "higher grade" of epoxy primer. DP has a great reputation as a great product.