Fill a bucket with warm water and numerous lemon slices. Soak your knee pads in this bucket for at least two to three hours. If you want to keep the water warm to help loosen the dirt, you can do it on the stove, but don't use a pot that you cook in.
Create a mixture of 1/3 cup of laundry detergent, 1/2 cup of normal detergent without bleach and 1/4 cup of white vinegar to wash the pads with in the laundry machine.
Remove the pads from the pot or bucket where they were soaking and turn them inside out. Wash them with the mixture in your washing machine on a delicate cycle with warm water.
Remove any remaining stain by rubbing Ivory soap directly on stain and rinsing with hot water.