At least I think that's what I used, it was called "VC Tar" from Interlux but does not appear to be available any more. The process was to grind off all the old bottom paint, then the gelcoat and get the keel down to bare, shiny metal. Following the VC Tar instructions, I used a special Interlux primer to coat the bare metal, truthfully it looked like the yellow etching primer you can get in a rattle can. I used the VC Tar putting on three coats as I recall. I used Interlux VC17 on the bottom after that and never had issues with the rust blooms like when we were using ablative bottom paint. It wasn't smooth like a baby's butt but was still smoother than the ablative with the constant rust blooms.

Other than it being the worst boat project I ever did, I don't recall having any issues after that. We subsequently sold that boat to a local sail builder, he keeps in the same slip (freshwater of course) where it has been since 1993 or 1994?