Porcelain and enamel are very hard, but harsh abrasives and acids will damage them. Abrasive cleaners make tiny holes in porcelain and enamel, then when dirt gets into these holes you will need to use more abrasive cleaner to clean your bathtub. After repeated uses, this abrasive cleaner will wear away the surface. A porcelain or enamel tub can last many years, if you are careful about how you clean it.
Scrub your porcelain or enamel tub with a solution of 15 ml (1 tbsp) of ammonia in 4.5 litres (1 gallon) of hot water. Rinse thoroughly after scrubbing. Turn on the exhaust fan, if you have one -- otherwise, open windows or doors to provide ventilation. Wear rubber gloves and a disposable mask to protect you from the ammonia fumes.
Place several paper towels on the bottom of tub. Pour undiluted chlorine bleach on the paper towels until they are completed soaked. Leave for 15 to 30 minutes. Rinse with warm water and dry with a towel.
Remove rust from the porcelain or enamel with a paste of 250 ml (1 cup) of borax and 50 ml (1/4 cup) of lemon juice. Put the paste on a sponge and rub over the rust. Rinse well.
Remove hard water stains by filling the tub with hot tap water and 750 ml (3 cups) of white vinegar. Let this stand for four hours. Rinse well.
Scrub your porcelain or enamel tub, after it has been cleaned, with full strength vinegar. The vinegar removes any soap residue left on the tub. Rinse it with cool water.
Polish the tub by rubbing it down with an old towel.
Do not use bleach on antique sinks. Damage is possible on older porcelain or enamel.