I was NOT referring to the POR-15 being snake oil. Only questioning its ability to stop the rust from returning.

An experienced body shop friend has always told me rust WILL return if the rusted area is not replaced. I'm hoping POR-15 will do what it claims.

The advertised claims just always seemed a bit... unscientific to me.

Yes:

Went to Sherwin-Williams catalog and found a 3 gallon size for bridges, etc. Corothane® I GalvaPac One Pack Zinc Primer. Not economical for our use.

OK, but your question was whether it could be bought off-the-shelf.

Anyhow, I just searched the Sherwin-Williams catalog myself and found a number of gallon sizes of Corothane in various colors.