The cleaning approach here depends on the tub's material. Enamel-finished steel or cast iron tubs are generally quite durable; those made of fiberglass or acrylics tend to scratch easily, and anything that has been scratched becomes progressively harder to keep clean.

 

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You can reduce the frequency of cleaning tubs and sinks by asking everyone in the household to dry the tub's surface after each use. Use either a dry terry-cloth towel or a squeegee. Drying the surface helps slow down the build-up of unsightly soap scum and ring-around-the-tub. It's also a good way to prevent the formation of mold or mildew on the tub or tiles.

Bon Ami, or a white scrubbing pad treated with detergent, will clean the no-slip surface treatment at the bottom of the tub; if soil persists, try baking soda and water with the pad.

Instead of buying a lot of expensive cleaners made for these surfaces, make a poultice of laundry detergent and water, which will attack hard-water stains as well as soap scum. Rinse well and dry to prevent formation of new water rings.

Avoid using heavily abrasive cleaners or pads on these surfaces, and don’t use acetone or other strong solvents on fiberglass or acrylics. Solvents can break down the plastics used in these tubs.

Reference:

 

Consumer Reports editors in How to Clean and Care for Practically Anything.