Muriatic acid, also known as hydrochloric acid, will cut through the thickest hard water deposits and soap scum, but it may also etch glass, discolor metal or cause serious burns. Before using it, try safer products, such as vinegar and commercial shower cleaning products. If you choose to use muriatic acid, exercise extreme caution. Wear boots and long pants, as well as a long-sleeved shirt. Dispose of plastic sheeting and cloths used during the cleaning process, and keep children away from the work area.
Unscrew the hinges that secure the door and take it outside. Muriatic acid produces strong fumes and can even cause explosions. Always use it outside. Cover a level, even surface, such as a patio with plastic sheeting and set the shower door on the plastic. Cover any surrounding plants with plastic to protect them from the acid.
Don rubber gloves and goggles. Pour 2 qts. water in a plastic container. Slowly add 4 oz. muriatic acid, and stir the mixture gently with a spoon. When muriatic acid combines with water, it produces heat and may even explode. Always add the acid to the water, rather than adding water to the acid. Use a plastic container because muriatic acid reacts with metal.
Dip a clean cloth in the acid and wipe it on the glass, taking care not to touch the metal trim. Rinse the acid away with a garden hose and allow it to dry. Reinstall the door in the shower.