Paint advice needed

I've been using SPI epoxy primer as sealer before base coat or single stage. Just thin it out with slow urethane thinner after mixing it with the activator. It works great. Just let it flash off for the proper amount of time, or just make sure you top coat it with your color within the recoat window of time. I love SPI's primers and Universal clear. Good luck to you!
https://www.southernpolyurethanes.com/