Ask three tradespeople what “mastic” means and you may get three answers. To some it’s any sealant in a cartridge. To others it’s the stuff around the outside of windows. In older buildings, especially in Scotland, it can mean traditional sand mastic pointed around stone window surrounds.
The words matter less than the product. A bath, a window frame, a brick movement joint and a skirting board all need different things from a sealant. Using the wrong one is one of the most common reasons we see joints crack, go mouldy or leak. This guide explains the main types in plain English, with a comparison table and a quick guide to which one your job needs.
Why the words get mixed up
Historically, “mastic” meant a thick, putty-like material used to fill and seal joints. Many older mastics were oil-based or bitumen-based and set fairly hard. Silicone sealants arrived later and took over many of those jobs because they stay flexible and resist water.
Today, people say “mastic” for almost any gun-applied sealant, and “silicone” for the white or clear sealant in bathrooms. Trade names don’t help either. When you talk to someone about a job, it’s more useful to ask which type of sealant they plan to use, and why.
Silicone sealant
Silicone is a flexible, water-resistant sealant that cures by reacting with moisture in the air. Once cured it stays rubbery, which is why it suits joints that move a little, like the joint between a bath and the wall.
- Sanitary (anti-fungal) silicone contains fungicide to help resist mould. It’s the right choice for baths, showers, basins, wet rooms and sinks.
- Low-modulus silicone is softer and stretches more easily, which suits joints with more movement, such as some window and façade joints.
- Glazing silicone is made to bond to glass and frames.
Two limits to know. First, most silicone can’t be painted. Second, new silicone does not bond to old silicone, so old beads have to be removed completely before resealing. We use Arbosil and Everbuild silicones, chosen for the joint.
Acetoxy and neutral-cure silicone
Silicones cure in different ways. Acetoxy silicone gives off acetic acid as it cures, which is why it smells of vinegar. It cures quickly and bonds well to glass and glazed tiles, but the acid can attack some metals and some natural stone.
Neutral-cure silicone doesn’t release acid and has little smell. It’s the safer choice next to natural stone, mirrors, some metals and many plastics, and it’s often used outside around frames.
Neither is “better” in every case. The right one depends on what the sealant touches.
Hybrid (MS polymer) and polyurethane sealants
Hybrid sealants, often called MS polymer sealants, sit between silicone and polyurethane. Many of them bond well to a wide range of building materials and many can be painted once cured, which makes them useful around frames, trims and joints that will be decorated. Check the product’s own data sheet before painting.
Polyurethane sealants are tough and are widely used for construction and floor joints, where wheels, feet and abrasion would wear a softer sealant. Some need a primer on porous surfaces.
These are the sealants most likely to be described as “mastic” around a building’s exterior.
Need it done properly? Mr Mastic covers the South East, London and Scotland, with a one-year guarantee on every job. Call or email for a quick quote.
Decorators’ caulk
Acrylic decorators’ caulk is a water-based filler for small gaps that will be painted: along skirting boards, architraves, ceiling edges and between plaster and joinery indoors. It’s easy to tool and takes paint well.
It is not a waterproof, flexible sealant. Used around a bath, a shower or an external frame it cracks, stains and lets water through. If the gap is in a wet area or moves noticeably, use silicone or a hybrid sealant instead. Our guide on decorators’ caulk vs silicone goes into more detail.
Sand mastic
Sand mastic is the traditional material for pointing the gap between a timber window or door frame and the stone or brick around it. It’s made from burnt (roasted) sand mixed with an oil binder based on linseed oil, applied by hand and shaped into a neat fillet.
You’ll find it on sash and case windows across Scotland and on period buildings in London and the South East. On those buildings it looks right in a way that silicone doesn’t. Our guide to sand mastic for sash and case windows explains how it’s used.
Fire-rated and intumescent sealant
Fire-rated sealants are tested to help stop fire and smoke passing through gaps in fire-resisting walls and floors, for example around pipes, cables and door frames. Intumescent sealants expand in heat to close the gap.
They must be installed to the manufacturer’s tested details for the gap size and the wall or floor. Ordinary silicone or caulk is not a substitute. The specification normally comes from a fire risk assessor, designer or building control.
Comparison table
| Type | Typical use | Flexible? | Paintable? | Watch out for |
|---|---|---|---|---|
| Anti-fungal sanitary silicone | Baths, showers, basins, sinks | Yes | Generally no | Won’t bond to old silicone |
| Low-modulus silicone | Window frames, moving joints | Yes, very | Generally no | Joint depth and backing matter |
| Hybrid / MS polymer | Frames, trims, general building joints | Yes | Often (check data sheet) | Product choice varies widely |
| Polyurethane | Construction and floor joints | Yes, tough | Often | May need primer |
| Acrylic decorators’ caulk | Indoor gaps to be painted | A little | Yes | Fails in wet or moving joints |
| Sand mastic | Timber frames in stone or brick | Slightly, hardens slowly | Guidance says don’t overpaint | Needs traditional skills |
| Fire-rated / intumescent | Fire-stopping gaps | Varies | Varies | Must follow tested details |
Which one does your job need?
- Bath, shower, basin, sink or worktop: anti-fungal sanitary silicone, with a food-safe product beside the sink where needed. See bathroom resealing.
- uPVC, aluminium or timber window and door frames: a weatherproof, flexible exterior sealant chosen for the frame and the wall. See window and door mastic.
- Sash and case windows in stone or brick: often sand mastic, especially on period and listed homes.
- Brick, render or cladding movement joints: a flexible exterior sealant with backing rod.
- Skirting boards in dry rooms: decorators’ caulk if it will be painted; flexible sealant in kitchens and bathrooms.
- Gaps in fire-resisting walls: a fire-rated product to the specification you’ve been given.
Quick answers
Is mastic the same as silicone?
Not exactly. Silicone is one type of sealant; “mastic” is a looser word people use for many sealants, and for traditional sand mastic.
Can I use bathroom silicone outside?
It’s better to use a sealant made for exterior joints, chosen for the frame and wall. Bathroom silicone is formulated for wet rooms, not weather and sunlight.
Can I paint over silicone?
Most silicone can’t be painted. If the joint must be painted, a paintable hybrid sealant or decorators’ caulk (indoors, in dry areas) is the usual answer.
Why does my bathroom sealant smell of vinegar?
That’s an acetoxy silicone curing. The smell fades as it cures.
Which sealant brands do you use?
We use Arbo (Arbosil) and Everbuild products, chosen for each joint. We’re an ARBOMaster approved applicator.
Related services
More advice
- Decorator’s Caulk vs Silicone: Which and Where
- Sand Mastic for Sash and Case Windows
- Window Mastic: When to Replace It and How
- What Is Fire-Rated Sealant? A Plain Guide
- All advice
This guide is general information about sealant work. It isn’t legal, fire-safety or structural advice; for those, speak to a qualified professional.