I finish sand with 400; 600 for the lighter metallics.......followed with a sealer coat just prior to paint
SPI epoxy with a little reducer as a sealer (it's all on the data sheet)....... other than a can of aerosol etch for little things , SPI epoxy is the ONLY primer I use
when I de-nib the base after a few passes, it also de-nibs the sealer coat
To use as a Paint Sealer: To use this epoxy as a paint sealer, mix it 1:1 with activator and reduce 10-50% with the proper temperature range urethane reducer (this is very important), induce for 30 minutes and spray with your base/clear gun. Spray one wet coat ONLY, let it sit 2 hours then apply paint. For sealing of a potential problem paint job, apply two coats of epoxy with proper flash times between coats and let it sit overnight before painting. You may basecoat over Epoxy Primer reduced as a sealer from 2-48 hours without sanding. After 48 hours scuff with a gray scuff pad or equivalent first.
https://www.southernpolyurethanes.com/_files/ugd/8ced3e_2e1da7dc5da34c94bb9c9704d8464b44.pdf