Coating hcast-ds321 Gray Duct Mastic Sealant 1 Gallon

Product Description

The COATING hcast-ds321 is a gray, lightly fibrous duct mastic sealant used to seal joints, seams, and connections in sheet metal and flexible ductwork systems. With a solids content of 65%, it delivers a durable, long-lasting seal after application. It dries to the touch in 1 to 2 hours and reaches full cure in 48 hours, allowing work to proceed on a reasonable schedule. It carries a UL Listed 181B-M rating, confirming it meets performance standards for closure systems on HVAC duct assemblies. HVAC technicians and sheet metal workers apply this product as part of duct sealing and air sealing work on both new construction and retrofit installations.

This mastic sealant is intended for use on low-pressure HVAC ductwork systems where a compliant, code-recognized seal is required. It is suited for sealing longitudinal seams, transverse joints, and connections between duct sections in residential and light commercial forced-air systems. The fibrous body gives it bridging capability across small gaps and irregular surfaces common in field-fabricated ductwork. It is typically applied by brush, trowel, or gloved hand directly to clean, dry duct surfaces.

Key Features

• UL Listed 181B-M rated for use on flexible and rigid duct closure systems
• 65% solids content for a dense, durable seal after cure
• Dry-to-touch in 1 to 2 hours, full cure in 48 hours
• Lightly fibrous formula bridges small gaps and surface irregularities
• Gray color for easy identification on installed ductwork
• Available in a 1-gallon container for field and shop use

Compatibility

Designed as a UL Listed 181B-M compliant mastic sealant for use on sheet metal ductwork, rigid fiberglass duct board, and flexible duct systems in residential and light commercial HVAC applications.

Installation Notes

Applied by licensed HVAC technicians or sheet metal workers as part of duct sealing work governed by local mechanical codes and ACCA or SMACNA standards. Shut down the air handling system before applying to ensure adhesion and prevent contamination of the mastic during the cure period. Apply only to clean, dry surfaces free of grease, dust, and loose debris. Do not thin with water or solvent, and protect applied mastic from freezing until fully cured.

Technical Specifications

• Color: Gray
• Texture: Lightly fibrous
• Solids Content: 65%
• Dry to Touch: 1 to 2 hours
• Full Cure Time: 48 hours
• Listing: UL Listed 181B-M
• Container Size: 1 gallon
• UNSPSC Code: 30141505

Standard Information

• Brand: COATING
• Manufacturer Part Number: hcast-ds321