Use Soft Scrub and Dish washer detergent put the detergent in the tub with a little water get a little brush and the little grounds get down in the little grooves in the bottom of the tub.(if you have the no slip surface)
Then take the soft scrub and wipe the tub down. Now you have a clean tub run some warm water, put the kids to bed, and take a nice long bath in your clean tub!
