wikiHow is a fantastic resource to use as a guide for using different products and processes when cleaning with natural products.
Caring for an acrylic tub is quite simple if one uses the correct products and cleans the tub after each use.
Natural products are preferred since using them is not hazardous to your health and they are easy to find. In fact, the products recommended here are most likely already in your kitchen cabinet.
Vinegar and baking soda along with some elbow grease will clean mold, mildew and soap scum.
Read about it here: How to Clean an Acrylic Tub
