Ask for “mastic” on a commercial job and you could get any one of a dozen products. Three families cover most of the joints a facilities manager or site team will see: silicone, polyurethane (PU) and hybrid, or MS polymer, sealants. Each has strengths. Picking the wrong one is a common reason joints fail early. This guide compares them in plain terms. Always check the manufacturer’s data sheet and the project specification before choosing.
Silicone
Silicone sealants handle ultraviolet light and weather very well, which is why they are widely used outside. Two broad types matter:
- Low-modulus neutral-cure silicone for facades, window perimeters and many movement joints. It stretches easily and stays flexible.
- Sanitary silicone with anti-mould additives for washrooms, kitchens and wet areas.
Most silicones cannot be painted. Silicone does not stick to old silicone either, so a reseal always starts with a full cut-out. Acetoxy silicones can attack some metals, stone and mirror backings, so neutral-cure grades are used for most building joints.
Polyurethane
PU sealants are tough and resist abrasion. That makes them a common choice for floors, concrete joints and joints that take foot or wheeled traffic. Some grades can be painted over. On the other hand, some PU products are less tolerant of long-term sunlight than silicone, and many need a primer on certain substrates. Check the data sheet for both points. For trafficked floors, see our warehouse floor joints guide.
Hybrid or MS polymer
Hybrid sealants, often sold as MS polymer, sit between the two. They bond to a wide range of substrates, often without a primer, and many are paintable. They are popular on refurbishment jobs where the joint meets mixed materials, or where the decorator needs to paint over the bead. Movement capability and weathering vary a lot between products, so don’t assume one hybrid behaves like another.
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Matching the sealant to the joint
A rough guide, always subject to the specification:
- External facade and window perimeter joints: low-modulus silicone, or a hybrid or PU where specified
- Washrooms and kitchens: sanitary silicone, or a hygienic sealant where food contact is possible
- Floor and car park joints: PU or another trafficable product
- Painted internal joints that move: a paintable hybrid or PU
- Internal gaps that don’t move and will be painted: acrylic decorators’ caulk
- Fire-resisting walls and floors: a fire-rated or intumescent sealant to the tested detail
BS 6213 gives guidance on selecting construction sealants, and BS EN ISO 11600 classifies them by movement capability. Our sealant specification guide goes further.
Questions to ask before you choose
Before signing off a product, ask how much the joint will move, what the two faces are made of, whether it will be painted, whether it faces sun and rain, and whether it carries traffic. Ask too whether a primer is needed and whether the colour matters. Those answers usually narrow the choice to one family quickly.
Fire-rated and specialist products
Fire-rated and intumescent sealants are a separate group. They are chosen for their tested fire performance in a specific detail, not for general weathering or movement, and they should only be used where the specification calls for them. Food-safe and hygienic sealants are another special case for kitchens and food areas. Polysulphide, a traditional sealant, is still found on some facade and trafficked joints. If you inherit a building with a mix of products, record what is where before planning any reseal.
Quick answers
Can I put polyurethane over old silicone?
No. Sealants generally don’t bond well to cured silicone, and silicone leaves a film behind. The old bead should be cut out and the joint cleaned back to sound, dry faces before any new sealant goes in.
Is MS polymer better than silicone?
Neither is better across the board. MS polymer is handy where paintability and adhesion to mixed surfaces matter. Silicone is often preferred for long-term exposure to sun and weather. The joint decides.
Do I need a primer?
Only where the sealant manufacturer specifies one for that substrate. Porous materials such as concrete and some stone often need one with certain products. The data sheet will say.
Related services
More advice
- Sealant Specification Guide for Contractors
- BS EN 15651 Sealant Classes in Plain English
- Sealant Joint Design: Width, Depth and Backing
- Adhesive, Cohesive and Substrate Sealant Failure
- 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.