Sealant is the general name for paste-like sealing and jointing compounds gunned from a cartridge to close joints and gaps against draughts and moisture. Once cured it stays flexible, so it can follow the movement between two elements without splitting. It is applied with a sealant gun; cartridges are typically around 290 ml.
Worth being clear about: sealant is a finish, not a structural or waterproofing layer. A bathroom floor is made watertight by a membrane under the tiles; the sealant in the corner is only the visible closing joint.
The main types
| Type | Properties | Typical use |
|---|---|---|
| Silicone | Very flexible, waterproof, cannot be painted | Sanitary work, glass, ceramics |
| Sanitary silicone | Silicone with a fungicide additive | Bath, shower, worktop |
| Acrylic | Paintable (depending on type after roughly half an hour), limited movement capability | Internal joints that will be painted, skirting and frame joints |
| Butyl | Stays permanently soft, tacky and strongly adhesive | Glazing, roof penetrations, concealed junctions |
| Polyurethane / PU adhesive | Strong, paintable, high adhesion | Structural joints, bonding and fixing |
How to get a joint that lasts
- Clean, dry substrate — remove all the old sealant (blade and sealant remover) and degrease with spirit or a cleaner
- Backer rod in deep joints — insert a round foam profile so the sealant bonds on two sides only and can stretch
- Mask — tape along both edges gives a straight, narrow joint
- Apply — cartridge at about 45 degrees, even pressure, work through in one pass
- Tool it — with a spatula or a wetted finger; for silicone, water with a little detergent helps
- Pull the tape straight away — before the sealant skins over
Points to watch
- Choose for the substrate and the job — adhesion to plastic, powder coating, natural stone or concrete varies a great deal. Natural stone often needs a dedicated sealant, as ordinary silicone can stain the edges
- Painting — silicone cannot be painted; if it has to take paint, use acrylic or PU
- Never bond on three sides — sealant that also sticks to the back of the joint cannot stretch and will tear
- Application temperature — most products want above 5 degrees; check the data sheet
- Cost — expect a few pounds or euros up to around ten per cartridge depending on type and quality; one cartridge usually covers several metres of a normal sanitary joint
Related terms
- Wall-floor joint
- Expansion joint
- Window frame
- Drip sill
- Damp-proof course
- Bitumen
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.