epoxy primers are ment to seal and protect the metal from moisture. Your sanding off your protection...... If you stay within the primers recoat window.... See your data sheet..... You can spray on a heavy coat of surfacer and then sand that. Your choices now are to fully sand the existing primer then reprime then apply the surfacer..... some people use a etching primer if the sand through is small so they don't have to reprime. You still have the problem of future sandthroughs. Put on a heavy coat of surfacer and sand that using a guide coat. You can get a better finish and hopefully avoid the sandhroughs.

Mike

1940 Chevy 1/2 Ton presently... Almost done
1940 Chevy Business Coupe... In pieces