DuraBlockâ„¢ Stain Blocking Paint Primer is a multi-surface interior and exterior water-based bonding primer that uses bio-based chemistry to replace toxic petrochemical ingredients without compromising quality. Use it to effectively prime surfaces that require a higher level of stain blocking, moisture blocking, rust inhibitor protection, and better overall bonding of the topcoat paint. The high solids formula with superior flexural strength enables better fill of hairline cracks and helps fill porous surfaces for better topcoat coverage.