Mix 1/2 cup of baking soda with enough water to make a thick paste -- the mixture should be about the same consistency as peanut butter. Moisten the stall walls with a spray bottle.
Spread the mixture over the shower stall walls. Allow the mixture to sit on the walls for about five minutes. Scrub the walls with a non-abrasive sponge or cloth.
Mist the walls with a spray bottle filled with white distilled vinegar. The walls will start to fizz. Allow the vinegar to soak into the walls for about five minutes.
Scrub the walls again to break up any mineral deposits or strip away discoloration. Rinse the walls thoroughly.
Make a solution using trisodium phosphate if the walls are still discolored. Mix 2 tablespoons to 1/4 cup of trisodium phosphate crystals into 1 gallon of hot water. Stir until the crystals dissolve. Scrub the walls with this solution and a sponge or cloth.