How to Use Mastic Sealant

Written by sal marco
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How to Use Mastic Sealant
Save money on heating and cooling costs by sealing air leaks. (glazier image by Greg Pickens from

Mastic sealant is an adhesive bonding agent that remains flexible even after drying. Mastic sealant has many uses on a wide variety of surfaces ranging from wood to glass to duct work. It stands up well to moisture-rich environments or dry environments. Used indoors or out, mastic sealant creates a barrier on seams and joints. Sealing joints, seams and gaps will reduce energy costs by creating a barrier from air leaks. Sealed joints will prevent warm or cool air from escaping or entering a home.

Skill level:

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Things you need

  • Degreasing dish soap
  • Rags
  • Caulking gun
  • Utility knife

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  1. 1

    Clean the surfaces that are to be sealed, using a mild degreasing dish soap and rag to remove oils, dirt, dust and grease.

  2. 2

    Dry the surface thoroughly with a clean dry rag or allow to air dry before applying sealant.

  3. 3

    Pull back the slide on a caulking gun until it clicks and locks. Insert the tip of a tube of mastic sealant into the circular end. Press the sealant tube down until it touches the base of the caulking gun. Slowly release the slide until it meets the bottom of the tube. Use a utility knife to cut the tip of the tube off at a 45-degree angle.

  4. 4

    Use the caulking gun to squeeze mastic along seams, around windows, in gaps or joined surfaces. Begin at an edge or corner joint and run the bead of sealant to the opposite edge or corner joint, covering the width and length of the seam or gap. If you are sealing a circular duct, run the sealant completely around until it meets back at the beginning.

Tips and warnings

  • Practice running a bead of sealant on a scrap board until you are able to handle the caulking gun easily.

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