Joint sealants (caulks) are often used as a quick fix to address a problem leak, but proper attention to material selection and the joint onto which it is applied can make the difference between a long-term repair and a recurring leak. See our BLOG entry on proper sealant joint preparation, design, and application http://blog.etc-web.com/?p=854.
The most widely used commercial-grade building sealants are made of either silicone or urethane. Other materials regularly enter the marketplace, many of which are specialty products for unusual/unconventional uses. Acrylic terpolymers comprise another family of sealants (and will be a topic for a future blog entry), but silicones and urethanes are mainstays in the construction trades.
Silicones tend to be longer lasting, are resistant to ultraviolet (UV) degradation, and most commonly used on non-porous substrates like metals, window-glazing and certain masonry projects. Paint will not adhere to silicone. Urethanes are normally less expensive and are paintable . They are often used on porous or natural materials like wood, masonry, concrete, and stone repair projects.
Sealants come in a wide range of colors, single and multi-component formulations, and non-sag or self-leveling viscosities. They’re also classified by elongation characteristics. For example, a rating of 50% means that the sealant in a half-inch-wide joint can stretch or compress up to one-quarter of an inch without tearing. There is no single sealant that’s suitable for every application and product selection is key to performance.
Surface preparation is essential to achieving a long lasting bond. Many sealant manufacturers require application of a designated primer to the substrate. Pull tests should be performed on a section of cured sealant to ensure adequate adhesion. In a standard pull test, the sealant should tear before it debonds from the substrate.
Silicone and urethane sealants do not stick to each other and should never be used where such a bond would be necessary to performance. When replacing sealants in a joint, the old materials must be completely removed.
Lastly, always tool the sealant into the joint. This not only imparts a finished appearance, it helps achieve a better bond to the substrate