What is mastic?
In UK construction and trade language, 'mastic' is often used broadly to describe joint sealants and the act of sealing. It can refer to several types of product including silicone, polyurethane, polysulphide and acrylic sealants. When someone asks for a 'mastic man', they usually mean a sealant applicator.
What is silicone?
Silicone is one type of sealant chemistry. It has excellent flexibility and water resistance, which makes it popular for bathrooms, kitchens and some external weather-sealing applications. Specialist silicones are also made specifically for glazing, façades and other external uses.
Where each sealant is typically used
As a general guide, different joints call for different products. The table below shows typical choices for common applications — the final selection always depends on the joint, the substrate and the manufacturer's guidance.
| Application | Typical sealant type | Why |
|---|---|---|
| Bathrooms and showers | Sanitary silicone | Flexible, waterproof and resistant to mould in wet areas |
| Kitchen worktops and sinks | Sanitary silicone | Waterproof seal that copes with regular cleaning and splashes |
| Window and door perimeters | External-grade silicone or hybrid | Handles UV, weather exposure and movement around frames |
| Expansion and movement joints | Low-modulus polyurethane or specialist façade sealant | Formulated for larger, repeated joint movement |
| Internal decorating gaps | Acrylic or flexible filler | Paintable, for low-movement gaps before decorating |
| Structural and glazing joints | Per specification or project drawings | Product is selected for the substrate, load and exposure |
Which sealant is right for the job?
The right product depends on the substrate, joint size, expected movement, exposure, compatibility with adjacent materials, and the manufacturer's technical guidance. Silicone is often suitable for wet areas and some external glazing, while polyurethane, polysulphide, hybrid or acrylic systems may be better for structural, high-movement or façade joints. We select the right sealant for each application rather than using one product everywhere.
Just as important as the product is the preparation. Old sealant should be fully removed and the joint cleaned before any new material goes on — a fresh bead over failed sealant usually fails again quickly.
If you are not sure which product a joint needs, a photo of the area and a note of the surface material and location is usually enough for us to advise and quote.
Frequently Asked Questions
Can you use silicone outside?
Some silicones are formulated specifically for external glazing and façade weather-sealing. The suitability depends on the particular product and its intended use, not the chemistry alone.