DEFY Stain Blocker For Roofs is a milky white liquid treatment that dries to a clear natural look. Spray on cleaned asphalt shingle roofs to form an invisible barrier that blocks the return of unsightly roof stains. Stains are effectively blocked from returning for up to three years depending on roof conditions and climate. Maintenance coats of stain blocker can be re-applied every 2-3 years or at the first sign of stain return to help block further staining. DEFY Stain Blocker For Roofs contains no acids, caustics, or bleaches and will not harm plants when used as directed.