If you apply epoxy primer over the bare metal, you don't need any Ospho. If you apply your high fill primer over the epoxy primer within a few days, you don't need to sand the epoxy.
Proper method would be: Sand to bare metal. Apply 2 coats of Epoxy Primer right on the bare metal. Use polyester filler to fill any small dents or imperfection on top of the epoxy primer. ( no need to sand back to bare metal for the filler) Within a week, apply 3 coats of High fill eurathane primer. Apply guide coat and block sand. Repeat with high fill primer if needed.
Always use a guide coat when block sanding. When you are sure that you are ready for your top coats reduce the Epoxy Primer as directed in P-Sheet and apply 2 coats. (This acts as a sealer). Then apply your top coats.
You will have a durable finish when done.
Frank
