Sealants and adhesives are among the most misused products in construction and domestic fit-out. The range available is wide, the chemistry genuinely differs between product types, and selecting the wrong one for the substrate or the environment leads to joint failure, debonding, or costly remediation. This guide explains the main chemistries, when to use each, and which products are worth reaching for on site or at home.
Understanding Sealant and Adhesive Chemistry
Not all sealants behave alike, and the active chemistry determines where and how a product can be used. The main types you will encounter in UK construction are:
Silicone — Highly elastic, excellent UV and moisture resistance, very long service life. Cannot be painted and will not accept paint adhesion once cured. Bonds best to glass, ceramic, and non-porous substrates.
MS Polymer (Modified Silane) — A hybrid technology that combines the flexibility of silicone with paintability and broader substrate compatibility. Bonds well to timber, concrete, plastic, and metals. Increasingly the default choice for trade applications where versatility matters.
STPU (Silane-Terminated Polyurethane) — Similar performance envelope to MS polymer, used in some premium hybrid sealant-adhesive products. Excellent adhesion without priming in most cases.
Acrylic / water-based sealants — Paintable and easy to apply, but far less flexible and less durable than silicone or MS polymer. Suitable only for internal use in low-movement joints — filling gaps around skirting boards before painting, for example. Not appropriate for wet areas or external exposure.
Polyurethane — Used in expanding foam products. Excellent gap-filling and insulating properties; high adhesion to masonry and timber. See the section on expanding foam below.
Understanding these distinctions avoids the most common mistake on site: using an acrylic decorators’ caulk in a wet room, or silicone in a joint that needs painting over.
Bathroom and Wet Area Sealants
For any internal wet area — baths, showers, sinks, and splashbacks — silicone remains the correct choice. It does not absorb water, withstands temperature cycling from hot and cold water, and resists the mould growth that would rapidly degrade an acrylic alternative in a humid environment.
Anti-mould formulations are worth specifying over plain silicone. UniBond Anti-Mould Silicone Sealant contains a built-in biocide that inhibits the black mould growth that commonly ruins bathroom sealant joints. It is a reliable, widely available product that performs consistently in domestic bathrooms and en-suites. Apply it to clean, dry, grease-free surfaces — even silicone with strong substrate adhesion will debond if there is any residue of cleaning products or old sealant left on the surface.
When removing and replacing a failed silicone joint, mechanical removal with a scraper is not enough. Use a dedicated silicone remover to break down any remaining film before applying fresh product, otherwise the new joint will peel within months.
External Weatherproofing Sealants
External sealant joints face a different set of demands: UV exposure, thermal movement, driving rain, and in many cases the need to take paint or a rendered finish over the top.
For expansion joints around window and door frames, movement joints in render, and sealing around external pipework or roof penetrations, a paintable, flexible sealant is required. Everbuild Weather Mate Sealant is formulated for exactly these conditions — UV-stable, overpaintable, and flexible enough to accommodate the seasonal movement common in timber frames. It bonds to most standard building substrates including uPVC, timber, masonry, and mortar.
The key external sealant rule: check the movement accommodation factor (MAF) on the data sheet. For wide joints or substrates with high differential movement, you need a low-modulus product with a MAF of 25% or higher. Applying a hard, low-movement sealant into a joint that moves will cause cohesive failure — the sealant splits down the middle — within one or two winters.
Hybrid Sealant-Adhesives
The most significant development in the sealant category over the past two decades has been the rise of hybrid MS polymer and STPU sealant-adhesive products. These do two jobs: they seal a joint against water and air infiltration, and they bond the substrates together with load-bearing adhesion.
CT1 Sealant and Construction Adhesive has earned a strong reputation in the UK trade, particularly for its ability to cure and bond in wet conditions. This makes it useful for sealing around wet-area installations where getting a completely dry surface is not always realistic, and for bonding substrates — shower trays, worktops, stone — where a standard grab adhesive would not hold. It is paintable, flexible, and compatible with a wide range of materials.
Sika Stixall Extreme Power Adhesive is another high-performance hybrid in this category, offering very high bond strength on most common building materials including glass, metal, timber, concrete, and most plastics. It is worth keeping on site when you are bonding dissimilar materials or working in conditions where priming would add significant delay to the workflow.
Both products represent a genuine advance over traditional sealants or standalone grab adhesives. The limitation is cost — these are premium products, and reaching for a hybrid when a standard silicone would do is unnecessary expenditure.
Grab Adhesives: When to Use Them
Grab adhesives are solvent- or water-based contact-style adhesives designed for bonding trim, panels, coving, and similar elements to walls and ceilings without mechanical fixings.
UniBond No More Nails Original is the best-known example and is adequate for interior light-duty applications — bonding lightweight skirting board sections, coving, wall panels, or mirror clips. It cures to a rigid bond and is not suitable for joints that need to accommodate movement.
For heavier sections, uneven surfaces, or where a higher initial grab is needed to hold a component in place while the bond develops, Evo-Stik Gripfill Original Adhesive is a stronger option. Its gap-filling properties make it more tolerant of slightly irregular or porous substrates such as rough plaster or uneven blockwork. Apply in spots or a continuous bead, press and pull the component away briefly to allow the adhesive to form a skin, then reposition and press firmly.
With any grab adhesive, surface preparation is the single biggest variable in bond quality. The surface must be clean, dry, structurally sound, and free from dust, wax, or release agents. Bonding to painted surfaces is generally lower-strength than bonding to bare substrate — if the painted layer itself is poorly adhered, the whole assembly will fail at that weak point.
PVA and Wood Glue
For timber joinery and woodworking, PVA-type adhesives are the default for most glued joints. Modern PVA wood glues are considerably stronger in tension than the timber itself when the joint is well-fitted, making the wood the weak point rather than the glue line.
Gorilla Wood Glue is a waterproof PVA-type adhesive with a longer open time than many cheaper alternatives, which is useful when assembling complex joints or large panel glue-ups where everything needs positioning before the adhesive begins to set. It sands cleanly once cured without gumming up abrasives — important if glue squeeze-out at the joint needs to be cleaned up before finishing. For external joinery or timber that will be exposed to persistent damp, specify a D3 or D4 rated waterproof adhesive rather than a standard interior PVA.
Expanding Foam as a Sealant
Polyurethane expanding foam is not a sealant in the conventional sense, but it performs a sealing and gap-filling function around window and door frames, pipe penetrations, and roof openings that no other product handles as efficiently.
Soudal Gun Grade Expanding Foam applies in a controlled bead from a foam gun, expanding to fill voids and bond to masonry, timber, and PVC. The gun-grade formulation gives significantly more control and less waste than a straw can, and it makes sense for any gap larger than about 20mm wide or any job involving more than two or three frames. Trim it flush once cured and cover the exposed foam face — UV degrades uncovered foam relatively quickly.
Note that standard low-expansion foam is not fire-rated. For fire-stopping applications around pipe penetrations through compartment walls or floors, specify an intumescent fire-rated foam product.
Application Tips
- Gun choice matters. A skeleton frame gun loaded with cartridge sealant is far more controllable than any caulking tube. Invest in a decent gun with a smooth, consistent trigger action.
- Cut the nozzle smaller than you think you need. A smaller bead is easier to tool than cleaning up an oversized one.
- Tool within the open time. Most sealants have a skin-forming time of five to fifteen minutes. Tool the joint with a wet finger or spatula before the surface begins to cure.
- Masking tape for clean edges. Apply tape either side of the joint before sealing, tool the joint, then remove the tape immediately before the sealant skins. This produces a clean, professional finish with no smearing.
Common Mistakes
Using acrylic caulk in a bathroom. It will fail within a year. Silicone only in wet areas.
Applying sealant over old sealant. New product will not bond reliably to old silicone. Remove it fully.
Not reading the data sheet. Movement accommodation, substrate compatibility, and overcoating times vary significantly between products. Two minutes with the technical data sheet avoids most failures.
Using a standard grab adhesive as a structural fixing. Grab adhesives are for trim and light panel work. They are not a substitute for mechanical fixings on anything load-bearing.
Summary
Sealant and adhesive selection comes down to chemistry, substrate, and environment. Silicone for wet areas, paintable hybrids for external joints with movement, MS polymer sealant-adhesives for demanding bonding and sealing in a single step, grab adhesives for interior trim, and waterproof PVA for timber joinery. Spend the extra time on surface preparation — no product performs well over a contaminated or structurally weak surface — and always check the manufacturer’s data sheet before applying in any application where failure would be costly to remedy.
