Bathtub cleaning

Does anyone know of a good bathtub cleaner? I have a 1960's porcelain enamel bathtub. I took all my Comet and Ajax to the county's hazardous waste disposal collection. Since I couldn't find a bathtub cleaner by SG, I bought one called Mrs. Meyer's Clean Day Surface Scrub, which is in a cannister similar to Comet or Ajax. It might work OK if you started with a brand clean tub and cleaned it every time you took a bath. But our tub is quite stained and dirty, and this doesn't even touch the dirt. Any suggestions rather than going back to the store and buying Comet or Ajax again?