Scuffing Epoxy Primer

The time window is for a wet on wet application, your first coat of primer paint or whatever gains its adhesion from mechanical grip (sand scratches) every coat after that gains adhesion due to chemical adhesion. Once the primer or paint dries it needs to have sand scratches back on it to gain mechanical adhesion. I don't suggest sanding epoxy primers including PPG or Omni epoxy's. They are NONE SANDING primers. They sand like bubble gum and clog paper and scuff pads. You should spray your epoxy then cover with a sandable primer filler. The other reason you shouldn't scuff epoxy is that you cannot gain proper sand scratching for the filler primer to adhere. A grey scuff pad (scotchbrite) is only 600-800 grit, and a red is about 320-400... far too fine for proper filler primer. I would suggest 180 then filler primer, possibly 220.

Another way to eliminate this is to just use a corrosion resistant filler primer like NCP-270. It has great corrosion resistant capabilities and has excellent filling. The only reason people suggest you use epoxy is it is great for corrosion resistance and has good adhesion qualities for almost any surface.