An etching urethane primer? Never heard of such a think? If you've got all of this metal sandblasted this would be the ideal texture for epoxy primer, some self etch primers are not recomended for use over blasted metal- trapped acid is a possibility.
I'd use two good coats of epoxy then either a polyester or urethane primer surfacer. Polyester primer works great in pitted areas or anywhere there is a good amount of fill needed. Hopefully Barry will jump in and identify the
urethane etch primer you've described.