Borax is a natural soap scum remover. Make a thin Borax paste and give it plenty of time to soak so it dissolves the crusty soap scum layer.
Use cold water when making the Borax paste. Borates only dissolve completely in hot water, not cold, so the Borax will retain a slightly gritty texture when mixed with cold water. The grit will help remove soap scum when it is time to begin scrubbing the soiled surface.
To prepare this natural cleaning product, mix 1 cup of Borax with 1 cup cold water, then mix in a few drops of heavy-duty dish detergent to cut through any oils in the soap scum. Then, apply the cleaner to the surface and let it work for 10 or 15 minutes before scrubbing.