Pick up a canister of abrasive cleaning powder. For thick buildup on porcelain surfaces, you’ll need to use something a little more heavy-duty. Use a product like Comet or Ajax that comes in powdered form. The small particles will be able to penetrate deeper into stains that have set up on the surface of the tub to remove them.
Powdered cleansers contain chemicals known as surfactants which have mild abrasive properties. This makes them much more effective for addressing hardened, stuck-on residue. Apply abrasive cleansers sparingly. For most jobs, a single canister be enough for a single thorough cleaning.
Stick with natural alternatives. Alternately, tough stains like rust and hard water residue can be treated with a mixture of hydrogen peroxide and cream of tartar. Blend the two ingredients together until they’re about the same consistency as cake frosting and spread them directly over the stains. After 10 minutes, buff the stains with a nylon brush or pumice stone until they’re erased completely.
Homemade concoctions like hydrogen peroxide and cream of tartar will be preferable for those who are wary of the health and environmental effects of chemical cleaning products. Hydrogen peroxide will also help treat age-related discoloration and lighten the finish of the tub.
Sprinkle the cleaning powder around the stained tub. A moderate dusting should be enough to deal with most messes. The powder won’t stick to the surface of the porcelain on its own, but when combined with a liquid it will form a paste that can be spread directly over stains. Be sure to cover the bottom of the tub, where accumulated mold can become a slipping hazard.