Heat one cup of vinegar in microwave. Add one tablespoon corn starch and two tablespoons of dish detergent. Mix and pour in spray bottle. Spray shower/tub, let sit an hour, wipe and rinse.
The vinegar dissolves hard water deposits, the detergent cleans and the cornstarch thickens the solution so it clings to the walls, allowing it to work better. It really does work!