Wipe the tub with a rag dampened with warm water and remove any debris or buildup layered on top of the porcelain surface.
Wipe the tub with a dry rag removing any additional moisture.
Dampen a fresh rag with a small amount of acetone. Place the rag over the top of an open acetone bottle, hold it firmly in place and quickly tip it upside down and back. The rag will dampen. If you are wiping the entire tub, place the rag in the sink and pour acetone on it until the entire rag is dampened.
Rub the rag dampened with acetone over the spot on the tub. Allow the acetone to dry for a minute and determine if the spot is lightening. Repeat until the stain is gone.