What Is Silicone Mastic?

In manufacturing, silicone mastic is a type of adhesive or sealant used in moisture-prone applications. It offers similar qualities to caulk, but provides a higher level of water resistance and anti-fungal properties than most caulking products. Silicone mastic also serves as an effective tool for sealing or adhering objects to smooth, non-porous materials like glass or tile. The mastic forms an extremely durable and strong bond on these and other hard-to-bond materials.

Plumbers and contractors often use silicone mastic in place of traditional caulk in commercial and residential bathrooms. The mastic acts as a bathtub sealant, preventing water from leaking into the walls or floor around the tub. It can also be used to caulk around bathroom countertops, and provides an effective tile adhesive on showers, bathroom walls, or floors.

Silicone mastic also allows contractors to install glass and windows on the exterior walls of a building. The mastic serves as a sealant between the window and the support frame, and also blocks rainwater from entering the building. Fire-rated silicone sealants can also be used to seal around exterior wall penetrations at cables, pipes, or exhaust systems.

Unlike many caulking products, this type of mastic is virtually waterproof. It also resists mold and fungal growth, making it ideal for bathroom and basement applications. This product is also relatively flexible and easy to install. It comes in a number of colors and finishes, and darker shades may help conceal dirt over time.

Because of its anti-fungal properties, this mastic should not be used in kitchens or food preparation areas due to the risk of contamination. The oil-based silicone used to make this mastic also prevents paint from adhering to the surface. Because the adhesive can't be painted, installers should choose mastic color carefully to match the project at hand. Some mastic products may stain or discolor over time due to excessive moisture, mold growth, or a number of other factors.

Silicone mastic must be installed over clean, dry surfaces. Users should cut away any old caulk or mastic products using a utility knife, then wipe the area to remove debris and dirt. Most tubes of silicone sealant can be applied using a standard caulking gun, though some may include built-in applicators to maximize convenience. Excess mastic must be wiped away quickly before it dries, and installers can shape the caulk joint as desired using shaping tools or a simple finger swipe.