Create a mixture of three-quarters cup powdered oxygen bleach and 1 gallon warm water. Stir the water until the oxygen bleach is completely dissolved.
Dip a plastic scrub brush into the mixture and work it into the rust stains. Continue to scrub at the rust stain until it is completely gone. If the rust marks persist, move onto the next step.
Pour 3 tablespoons cream of tartar, 2 tablespoons table salt, 1 tablespoon lemon juice and 3 tablespoons water into a plastic bowl. Stir the ingredients until they are well-incorporated.
Spread the homemade cleanser onto the difficult rust stains. Allow it to remain for 10 to 15 minutes before wiping it away with a soft cloth.
Rinse the acrylic shower stall with warm water and dry with a soft towel to prevent streaks.