What is MS polymer or hybrid sealant, and when is it worth the extra?

For decades the choice was silicone, which is waterproof and unpaintable, or acrylic caulk, which is paintable and not waterproof. Hybrid polymers broke that trade-off. They are worth understanding properly because for a lot of household jobs they are simply the right answer, and because a single tube covers work that would otherwise need three.

Step by step

1. Understand what it is chemically.A silane-modified polymer that cures by reacting with moisture in the air. It contains no solvent to evaporate, so it barely shrinks; no acetic acid, so it does not attack mirrors, stone, copper or concrete; and no isocyanate, so it is not a respiratory sensitiser in the way polyurethane is.
2. Know the four properties that make it useful.It bonds to almost every common substrate, including damp ones and several that silicone will not touch. It stays genuinely flexible with a high movement capability. It accepts paint, which silicone never will. And it resists ultraviolet, so it survives outdoors.
3. Use it where a joint has to be both painted and waterproof.This is the case it solves that nothing else does. An external window frame in a painted reveal, a threshold, a joint between rendered masonry and painted timber. Acrylic will crack there and silicone cannot be painted.
4. Use it where the two materials are different.Timber to brick, uPVC to render, metal to masonry. These joints move by different amounts and the high movement capability of a hybrid is what copes with that.
5. Use it as an adhesive as well.The adhesive versions of the same chemistry — sold as hybrid grab adhesive or adhesive-sealant — bond panels, mirrors, trims and cladding to almost anything, cure without needing a porous surface, and stay flexible. This is why one cartridge covers so much.
6. Know where silicone still wins.In permanently wet sanitary joints — around a bath, a shower tray, a sink — sanitary silicone with a fungicide is still the better choice, and it tools to a finer finish. Silicone is also cheaper and more available.
7. Test paint compatibility before painting a whole run.Hybrids are paintable but not with everything: some paints stay tacky over them and some crack. Apply a short bead, let it skin, and paint a test patch with the paint you will actually use.
8. Apply and tool it slightly differently.It is stiffer than silicone and more inclined to string. Cut the nozzle to suit, gun it steadily, and tool with water or a proper tooling solution — soap solution works less well on hybrids and can leave a film.

Tips

• The names are inconsistent — MS polymer, hybrid, SMP, silane-terminated polymer, 'silicone-free sealant'. Read the description on the cartridge rather than trusting the name.
• Because it cures with atmospheric moisture, a very deep or fully enclosed bead cures slowly from the surface inwards, exactly like silicone. Backer rod and a sensible depth still matter.
• It is the sensible choice for anything near a mirror, a natural stone worktop, a copper pipe or fresh concrete, all of which acetic silicone attacks.
• Overpainting reduces its flexibility, because the paint film is stiffer than the sealant. On a joint that moves a lot, an unpainted hybrid outlasts a painted one.
• The adhesive versions grab strongly but not instantly. Support the piece for the first day rather than assuming instant hold.

Alternatives

• Sanitary silicone: Better in a permanently wet joint, contains a fungicide, tools to a finer finish, cheaper. Cannot be painted and acetic grades attack mirrors, stone, copper and fresh concrete.
• Acrylic decorator's caulk: Cheap, easy to tool, takes paint perfectly and disappears under it. Low movement capability, shrinks as it dries, and fails outdoors or anywhere wet.
• Polyurethane sealant: Cures hard and tough with good abrasion resistance, so it suits thresholds and joints that are walked on. Harder to tool, and it contains isocyanates.
• Butyl and non-setting mastics: Never cure at all, so they stay permanently plastic. Used for bedding and lap joints in roofing and glazing rather than as an exposed bead.

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