If there is mold or mildew on the surface it needs to be removed before painting. The additive is for growth of mildew on the surface of the paint.
To remove mold/mildew wash the surface with a solution of 3 quarts water, 1 quart bleach and 2oz. of a liquid non ammoiniated detergent. Apply this to the surface and allow to remain wet for at least 15 minutes and rinse with clear water.