If you have an older tub and you're not afraid to mess it up or lose nothing in principle, use cleaning powders from any manufacturer. They are quite cheap, so I buy a few packages and tools, iron brush. Wet the tub and sprinkle over the entire surface scouring powder. Wait 10-15 minutes and active movements go with steel brush on the walls of the bath. The rust needs to come clean, and the bath start to Shine.
New bath better to carry out such experiments. For cleaning suitable means, with a creamy base. Of course, they also have grit, but it is somewhat smaller and does not damage the surface plumbing. Use a soft sponge and cream, do not attempt to clean the tub with steel brush, it will damage the surface and rust will start to accumulate much faster.
Sold in stores and special sprays to clean sanitary ware, which is specially designed not damage the smooth surface. Spray them wet the walls of the bath and wait about 10 minutes. Wipe the surface with a damp cloth or sponge and thoroughly rinse tool warm water.
There is a way for the lazy Housewives. Enter night full tub of hot water and add any oxygen bleach, but the concentration of funds should be higher than for bleaching things about twice. In the morning drain the water, and you will see the amazing picture - brilliant and white bath. Is bleach suitable citric acid, but this volume of water will need at least 30 bags. You can use vinegar, but the smell will penetrate the walls of the room and spread throughout the house, so the use of acid is undesirable.
The more you clean the bath as a preventive measure, the less likely that it formed stands of contamination.