Instructions. 1  Baking soda Baking soda is the best household item you can use to get the accumulated gunk off your old bathtub. 2  Mix two parts of baking soda and one part of hydrogen peroxide in a small glass bowl. 3  Apply this paste on your enameled bathtub or sink and let it set for about 30 minutes.nstructions. 1  Baking soda Baking soda is the best household item you can use to get the accumulated gunk off your old bathtub. 2  Mix two parts of baking soda and one part of hydrogen peroxide in a small glass bowl. 3  Apply this paste on your enameled bathtub or sink and let it set for about 30 minutes.