Step-by-step

1
Turn the system off and wipe dust and grease off the metal duct joints so mastic can grip.
2
Wrap any wide seams or gaps with fiberglass mesh tape to bridge them.
3
Brush a thick, generous layer of mastic over every seam, joint, and connection.
4
Work the mastic into the metal so it fills pinholes, covering the mesh completely.
5
Pay extra attention to take-offs and elbows where leakage is worst.
6
Let it fully cure before running the furnace again.

Common mistakes

• Using duct tape instead of mastic (it dries out and peels)
• Applying mastic over dusty, greasy metal
• Skimping on thickness so it cracks

Maintenance & next steps

Re-check the seams every couple of years and touch up any cracks.

When to call a professional

For sealing ducts buried in finished walls or if you suspect asbestos tape on old ducts.

Editor's note: This is instructional content for How to Seal HVAC Ductwork Seams with Mastic Paste. Replace with your own firsthand experience and photos before publishing.

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