Butyl mastic is a rubber-based sealant suitable for outdoor use on concrete, cinder block, brick, and stone. It is difficult to work with and requires patience and paint thinner for cleaning. It needs to contact every surface it is sealing, and excess should be removed before hardening. Practice beading before applying.
Butyl mastic is a rubber-based mastic that works especially well in outdoor applications. It is suitable for application to concrete, cinder block, brick and stone. It does a good job as a sealant when used on gutters, flashings, chimneys and aluminum siding. The bonding ability of this mastic in wet conditions makes it an especially good choice for use underground and on roofs.
Butyl caulk isn’t as easy to work with as other forms of caulk, such as silicone, so it’s typically only used in areas where other types of caulk wouldn’t be effective. It takes longer to cure than other types of caulk. When applying, it can be difficult to make an even application. Often the mastic becomes stringy during application. It has a tendency to shrink as it cures, which may make it necessary to reapply caulk to fill in uncovered spots after the curing process.
Cleaning after using this type of putty is more difficult than with other types of putty. Requires paint thinner for cleaning. Although silicone and other more forgiving caulks are difficult to keep clean during application, butyl caulk requires even more patience. The caulk will exit the end of the tube about 30 percent wider than the tube opening, so start with a small hole and enlarge the opening as needed.
For best results, remove all excess caulk before it hardens. Wiping off the excess with your finger can be second nature, but butyl caulk doesn’t rub off the skin easily, leaving a sticky residue that will end up on the trim, stone, or anything else in the area. Have a cloth moistened with paint thinner handy to run along any areas that need cleaning while applying the caulk.
Another important consideration when working with butyl caulk is that it needs to contact every surface it is sealing, including every side and bottom. Given the amount of shrinkage that can occur when applying the caulk, it makes sense to apply the caulk lightly during the initial application. At the end of the polymerization, remove the excess with paint thinner. Even the smallest gap between the caulk and the other surface will allow moisture to enter.
Practice beading a few times before applying caulk to the surface that requires sealing. People experienced with other types of caulk should also practice laying a bead with butyl caulk. It is more difficult to work with than other types of mastic, and the areas where it is used, such as around skylights and fireplaces, make frequent reapplications undesirable.
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