Use tape dams and backer rod to stop sealant from leaking out of the ends of a crack before you pour.

When preparing a crack for sealing, clean the joint thoroughly with a stiff brush and then with a solvent like rubbing alcohol to remove dust, oils, and loose debris so the sealant adheres. Measure the crack depth and select backer rod that is slightly larger than the gap so it compresses snugly into place at a depth that leaves the correct sealant thickness above it, generally about one half of the joint width for optimal elasticity. Install the backer rod by pressing it in with a blunt tool or a putty knife without stretching it; this creates a firm internal stop and prevents three sided adhesion which causes premature failure. For the ends, apply a tape dam using a high tack, tear-resistant tape such as Gorilla tape, pressed flat against the substrate to create a short temporary barrier; run the tape a half inch to one inch beyond the crack edges on both sides and overlap slightly for a solid stop. When you are ready to pour or gun the sealant, work from one end to the other in a continuous bead so you do not trap air, and tool the surface immediately with a wetted spatula or joint tool to force the sealant into contact with the faces while keeping the tape dam intact. Once the sealant skins over but before full cure remove the tape carefully at a low angle to leave a clean edge and avoid pulling on the uncured material.

Common mistakes are cutting the backer rod too small which allows sealant to pass underneath; fix this by choosing a rod diameter one quarter to one half size larger than the gap so it compresses and seals. Another issue is using low tack or paper tape that soaks through or lifts when sealant runs against it; use a high duty waterproof tape and press it firmly to make a true dam. A third problem is overfilling the joint and then removing the tape too early which smears and pulls the bead; tool the sealant to the desired profile and wait until the surface has formed a skin before peeling the tape away slowly.

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