Silicone vs MS Polymer vs Polyurethane Sealant: How to Choose?

Quick answer: These are the three dominant elastic construction sealants, and each wins in a different situation. Silicone offers the best UV and weather durability and is the only choice for glass and structural glazing — but it cannot be painted. MS polymer (modified silane) is the versatile all-rounder: paintable, low-odour, isocyanate-free, bonds to most substrates without primer, and won't stain natural stone. Polyurethane delivers the highest mechanical strength and abrasion resistance — ideal for trafficked and concrete joints — but degrades under sunlight unless it is painted over.

What are the three sealant families, in one line each?

• Silicone — a moisture-curing silicone rubber; unmatched UV stability and temperature range, but not paintable. Comes in acid-cure and neutral-cure types (see our guide on neutral vs acid cure).
• MS polymer — also called modified silane, silyl-modified polymer (SMP) or hybrid polymer; a polyether backbone with reactive silane ends. Combines silicone-like weathering with polyurethane-like paintability, minus the acetic acid and minus the isocyanate.
• Polyurethane (PU) — a high-strength elastic sealant prized for toughness and abrasion resistance, widely used on concrete and horizontal joints; needs UV protection (paint) for long exterior life.

When should I choose silicone?

Choose silicone when weather exposure, glass, or temperature extremes dominate the joint:

• Glass and glazing — silicone bonds to glass reliably and is the standard for window and façade glazing.
• Structural glazing — two-part structural silicone is the only chemistry approved for structurally bonding glass to framing; MS and PU are not used here.
• Long-term exterior weather seals — silicone has the longest service life of the three under sustained UV and weathering.
• High and low temperature extremes — silicone keeps its elasticity across the widest service temperature range.
• Sanitary joints — mould-resistant silicones are standard for kitchens and bathrooms.

The trade-off: silicone cannot be painted, and silicone residue can cause paint to bead up (fisheyes) on adjacent surfaces. For natural stone, use a non-staining stone-grade silicone (see our substrate compatibility guide) — or switch to MS.

When should I choose MS polymer (modified silane)?

Choose MS polymer when you want one product that does many jobs safely, especially where painting or substrate variety is involved:

• The joint will be painted — MS accepts paint after cure, unlike silicone.
• Mixed or porous substrates — MS bonds well to timber, concrete, masonry, metal and most plastics, often without primer.
• Natural stone — MS does not stain porous stone, making it a clean alternative to standard silicone on stonework.
• Low odour / solvent-free / isocyanate-free needs — relevant for occupied interiors and for health-and-safety-sensitive sites.
• Bonding plus sealing — MS polymers generally develop higher initial grab than silicone, useful where the sealant also has to hold parts together.

The trade-off: for the most extreme UV exposure, the highest movement classes, and glazing, silicone still leads.

When should I choose polyurethane?

Choose polyurethane when mechanical toughness is the priority:

• Trafficked and horizontal joints — floor joints, pavements, loading areas; PU resists abrasion and tearing better than silicone or MS.
• Concrete and masonry construction joints — PU bonds strongly to cementitious substrates and is a long-standing standard there.
• Joints that will be painted and are not in harsh sun — PU is paintable.

The trade-offs to plan around: PU degrades under prolonged UV (chalking/yellowing) unless painted, so it is less suited to exposed façade weather seals; and many PU sealants contain isocyanates, which carry handling and ventilation requirements that silicone and MS do not.

Can each one be painted over?

This single question decides many real projects:

• Silicone — no. Paint will not adhere to cured silicone, and silicone contamination causes coating defects nearby. If a joint must be painted, do not specify silicone.
• MS polymer — yes. Paintable after cure; confirm compatibility with the specific paint on a test area.
• Polyurethane — yes. Paintable, and for exterior PU joints painting also adds valuable UV protection.

Which sealant lasts longest outdoors?

Under sustained sunlight and weather, the durability order is generally silicone > MS polymer > polyurethane. Silicone's siloxane backbone is inherently UV-stable, which is why high-grade weatherproofing and curtain wall sealants are silicone. MS polymers weather well and sit in the middle. Unprotected polyurethane is the most UV-sensitive of the three and is usually painted when used outdoors.

Comparison table

Relative cost varies significantly by grade and regional market and is not shown; request current pricing for the specific grade you need.

FAQ

Q: What is MS polymer sealant, in plain terms? A: MS (modified silane) polymer is a hybrid sealant that pairs silicone-like weather resistance with polyurethane-like paintability. Chemically it is a polyether backbone capped with reactive silane groups, so it cures with moisture like silicone but releases neither acetic acid nor isocyanate. It is also sold as SMP or hybrid polymer.

Q: Is MS polymer better than silicone? A: Not universally — it depends on the joint. MS wins when the joint must be painted, spans porous or mixed substrates, or must avoid staining stone. Silicone wins for glass, structural glazing, the harshest UV exposure, and the widest temperature extremes. They are complementary, not ranked.

Q: Can I paint over silicone sealant? A: No. Paint does not adhere to cured silicone. If a joint must be painted, use MS polymer or polyurethane instead, and keep silicone away from surfaces to be coated, since residue causes fisheye defects.

Q: Which sealant is best for concrete? A: For concrete, both polyurethane and MS polymer perform well and bond strongly. Polyurethane is the traditional choice for trafficked and structural concrete joints; MS is a good paintable, isocyanate-free alternative. Acid-cure silicone should not be used on concrete because it is alkaline.

Q: Which sealant should I use on a façade exposed to strong sun? A: Silicone, for its UV durability and long service life. Reserve unprotected polyurethane for joints that are painted or sheltered, since prolonged sunlight degrades it.

Q: Are these three sealants compatible with each other in the same joint? A: They should not be applied over one another wet or relied on to bond across different cured chemistries. When replacing one with another, remove the old sealant fully and prepare the joint before applying the new one.

The information above is provided for general reference. For coated, painted or unusual substrates, conduct an adhesion test before full application. For project-specific recommendations, contact our technical support team.