To seal a rubber gym floor, use a sponge mop that has been dipped in a rubber floor finish and sealer. Do not dilute this solution. Apply the solution evenly with a light coat, being sure to cover all areas of your rubber floor.  Sealing your rubber gym floor is a good idea. It will provide you with an extra layer of protection against disintegration and bacteria absorption. However, you should not seal a floor covering that is made from recycled rubber. Doing so has been shown to speed up the deterioration of the flooring.