#7 Mastic Air Duct Sealant (1 gallon) #7 Mastic Low to High Velocity Air Duct Sealant. Color: Gray Premium Grade Duct Mastic That Remains Flexible & Saves Energy. UV & Weather Resistant, Water-Borne, & Non-Flammable. Easily Applied By Brush, Trowel, Putty Knife, or Palm. Save Energy - Seal Your Air Ducts & Crawl Spaces. Sheet Metal, Fiberglass, &amp; Flexible Air Ducts Register Boots Vents &amp; Roof Caps Plenums Thermal Insulation Crawl Space Vapor Retarders