Combine water and baking soda in a container to form a paste. Only a small amount will be needed to spread over the stain, so 3 tbsp. of baking soda and a few drops of water will be enough. Measure the amount of water needed by watching how thick the mixture gets while stirring it. Mix this with a dowel or with your hands until it is thick and will spread easily.
Spread the paste onto the stained area and allow it to sit for about 10 minutes. Wipe the paste off with a dry, soft cloth.
Combine 1 tbsp. of TSP in a 2-gallon jug with 1 gallon of water. Use the other dowel to stir the mixture until well-combined.
Spread the mix over the stained area and allow it to sit for no more than one minute. Wipe it off with a dry cloth, and rinse the area immediately. If left on for a long period, it can cause discoloration on the ceramic, fiberglass, or tiled bathtub.