Examine the art and take note of which areas are affected by the mildew.
Spray the mildew removal product liberally on the parts of the art that contain mildew.
Scrub these sections vigorously, being sure to use short strokes. Clean the art in sections, moving on to a new spot after each section has been cleaned.
Spray down the art with a water hose after mildew removal product has been used. This will get rid of any excess product.
Allow the art time to air dry. Since it's set up outside, this will occur naturally.