When a Caulking Gun Works Best
Sometimes spreading mastic with a brush does not provide good control over the mastic applied. Where access is tight and appearance is a concern, a caulking gun works best. RCD #6, #8 and #9 Mastic are available in 11oz. caulk tubes. See Duct Sealing Tips, Where to Look for Leaky Ducts.

Mastic
Use UL 181A-M and UL 181B-M Listed Mastics. RCD Corporation® #6, #8, #9, and PS-1 Mastics® are UL 181A-M and B-M listed. RCD Mastics can be applied by gloved hand, brush, trowel or caulking gun. Insulation can be installed over mastic that is still wet.

Fiberglass Reinforcing Membrane
This product is designed to be used with mastics. It reinforces the mastic when there is a gap of 1/4 inch or more. First apply a thin layer of mastic, then place the membrane in this layer of mastic and add another layer of mastic over the top so the membrane is completely covered. 

Duct Supports
Flexible ducts can easily be pinched by wire or twine supports so air flow is restricted. The best support material is woven polypropylene strap that comes in various widths - the wider the better. Sometimes called "webbing strap," this material is wide enough so it won't bite into flex duct. For convenience, use it for both metal and flex duct support.