Hard water stains and harsh cleaning solvents are the most likely causes of a yellowing acrylic bathtub. Eliminating hard mineral deposits and using a non-abrasive cleaner is the best way to remedy this problem.
Acrylic is a nonporous plastic commonly used in kitchen and bathroom sinks and tubs. This synthetic material is durable and easy to clean.
Cause #1: Hard Water
Hard water is common throughout the U.S. and is especially prevalent in homes that draw water from wells. Minerals in this water create limescale deposits that naturally yellow acrylic surfaces.
Cause #2: Bathroom Cleaners
Some bathroom cleaners are too harsh for acrylic surfaces. Consult the manufacturer of your tub for approved cleaners to ensure you don't permanently damage the acrylic lining.
Remove yellow stains caused by hard water deposits with Lime-Away or a vinegar and water solution. A mild soap and water solution also works for general cleaning. If the yellowing is the result of trying to clean the tub with a harsh solvent, it might have caused irreparable damage to the acrylic surface.
In you have hard water, install a water softening system in your home. There are a variety of systems available. Use mild bathroom cleaners made especially for acrylic bathtubs. A white vinegar solution or a store-bought detergent made for acrylic or fibreglass tubs are both good options to prevent discolouration.