Masked the galvanized parts, and then prepared and painted the tub interior today. There's no way the tub surface will ever look as if it just left the factory, due to the dents and scratches of carrying a load of cinder blocks and assorted junk as ballast weight while snowplowing for decades, but after sanding and three coats of paint, it does look a lot better!
Masked:
Painted. The surface will be checked tomorrow, and if all's well and no touch-ups required, the masking can come off.