Choosing the proper joint design and application procedure is as important as choosing the right exterior substrate for your project. At Sealant Sales, we regularly consult onsite with architects and engineers as well as general contractors and specialty contractors using the following guidelines:
Dow Corning has found that a few underlying principles are critical to consider in virtually all joint designs using silicone sealants. When considering the design of weatherproof joints, the following basic points must be addressed:
- In all cases, a minimum depth of 1/4″ (6 mm) sealant/substrate bond is necessary to ensure adequate adhesion.
- In most cases, a minimum width of 1/4″ (6 mm) opening is necessary to ensure that sealant applied from a caulking gun will flow into the sealant joints. NOTE: In some cases where the sealant is used simply as a non-moving bedding compound and is applied to one substrate before both substrates are pressed together, thinner joint dimensions are acceptable.
- One-part silicone sealants require atmospheric moisture to fully cure. Therefore, the sealant joint must be designed to ensure that the sealant is not isolated from the air
SEALANT APPLICATION PROCEDURE
It is critical that the sealant fills the entire joint or cavity and firmly contacts all surfaces intended to receive sealant. If the joint is improperly filled, good adhesion will not be achieved, and sealant performance will be weakened. To obtain full adhesion, sealants require a clean, dry, frost-free surface. Although silicone sealants have excellent wide-temperature gunnability, the practical application temperature can be dictated by frost formation on the joint edges, which can begin to occur below 4°C C (400°F). To assist in the drying of a frost-containing joint, a water-soluble solvent such as IPA should be used. Sealant should be applied as follows:
- Masking tape should be used to keep excess sealant from contacting adjacent areas where it is not intended, to ensure an aesthetically pleasing job.
- Apply the sealant in a continuous operation using a caulking gun or pump. A positive pressure, adequate to fill the entire joint width, should be used. This can be accomplished by “pushing” the sealant ahead of the application nozzle. Care must be taken to ensure complete fill of the sealant cavity.
- Tool the sealant with light pressure before a skin begins to form (typically 10 to 20 minutes). Tooling forces the sealant against the back-up material and the joint surfaces. Do not use liquid tooling aids such as water, soap or alcohols. These materials may interfere with sealant cure and adhesion and create aesthetic issues.
- Remove the masking tape before the sealant skins over (within about 15 minutes of tooling).