I am pretty new to this, only doing it for about a year on parts.
I go directly to base over the SPI epoxy primer, either sand it down to 4 or 600
or try to lay a slick reduced by 20% epoxy primer to act as sealer and base on that. it is probably the best way to go.. let the epoxy cure for a least a couple of hours, or best, overnight. then just tack and go. the primer comes in 3 colors, so choose your weapon..