E-Coat on new fenders

You need to read the product sheet for the epoxy you are using. Every manufacturer is going to suggest what they recommend for THEIR product to be sprayed over.

SPI Suggests 80 grit.

PPG Suggests for DPLF 80-180.

Sikkens EPII epoxy suggests 180-320

In my experience the beauty of epoxy is that it sticks to anything.

People tend to make the mistake of thinking a coarse grit paper will strip a panel faster, therefore they use that. When in reality in most cases 180 will strip a panel quicker, leave a scratch in the metal sufficient enough for material to stick and not leave a scratch so coarse that it can never be removed from metal again.. like 24,36,80....