Sealants: Silicone vs PU vs Acrylic

Sealant is one of the cheapest materials in a house and one of the most common reasons for complaints. The black line of mould along a shower screen, the crack that opens every winter between the window frame and the plaster, the yellowed bead around a white basin, the seepage stain below a balcony door — almost all of them trace back to a tube of sealant that was the wrong type, put in the wrong joint, or applied the wrong way. Most site teams carry one cartridge of "silicone" and use it for everything.

The three families found on every Indian hardware shelf — silicone, polyurethane (PU) and acrylic — are not interchangeable. They differ in how much movement they can take, whether they stick to concrete or glass, whether they can be painted, how they behave in sunlight and how they resist water and mould. A sealant that is perfect around a shower tray will fail in a floor expansion joint; one that is perfect under paint at a skirting will crack on an external window frame in the first monsoon.

This article explains how each sealant works, compares them on the properties that matter, shows where each belongs in a home with dimensioned joint details, and gives indicative October 2026 prices per cartridge. It also covers the newer hybrid (MS polymer) sealants and the step-by-step application that decides whether any sealant lasts.

From putty to polymers: a short history

For centuries, builders sealed gaps with whatever was sticky and soft enough: lime and plant mixtures, bitumen and pitch, oakum and tar in boats. Glazing putty — a paste of chalk and linseed oil — held window glass in timber frames from the eighteenth century onward and is still used in heritage restoration. These materials worked for small, slow movements but hardened and cracked with age.

The twentieth century brought synthetic rubbers and polymers. Polysulphide sealants were among the first high-performance elastomeric sealants and were widely used in curtain walls and concrete joints from the mid-twentieth century. Silicone sealants, based on a silicon–oxygen backbone, followed and became the standard for glazing and sanitary joints because of their resistance to sunlight and water. Polyurethane sealants took over many concrete and floor joints thanks to their toughness and paintability, and water-based acrylic sealants became the decorator's gap filler. More recently, hybrid sealants based on silane-modified polymers — often sold as MS polymer or hybrid sealants — have combined some of the strengths of silicone and PU. Today, international standards such as ISO 11600, EN 15651 and ASTM C920 classify building sealants by the movement they can take and by their intended use.

How sealants work

A sealant is a paste that is gunned into a joint, sticks to both sides, and cures into a flexible rubber. Its job is to keep water, air and dust out while the two sides of the joint move. Joints move for many reasons: concrete and masonry expand in heat and shrink in cold, aluminium and uPVC frames expand far more than the masonry around them, timber swells in the monsoon, buildings settle and slabs deflect. A sealant must stretch and compress with that movement without tearing or losing grip.

Movement capability is the key number. It is the percentage by which a sealant can stretch and compress, relative to the joint width, over many cycles without failing. A sealant rated at ±25 per cent in a 10 mm joint can accommodate the joint opening to 12.5 mm and closing to 7.5 mm. Rigid fillers such as cement, grout and plaster have essentially no movement capability, which is why a cement-filled gap between a frame and a wall cracks at once. Sealants are also described by their modulus: low-modulus sealants are soft and stretch easily, putting little stress on weak edges such as concrete or stone; high-modulus sealants are stiffer, suited to glazing and joints where strength matters more.

The three families in detail

Silicone

Silicone sealants cure by reacting with moisture in the air to form a very flexible, water-resistant rubber that is stable in sunlight and over a wide temperature range. They bond strongly to glass, glazed tile, ceramic, enamel, anodised and powder-coated aluminium and many plastics. Two cure types matter. Acetoxy silicones release acetic acid as they cure — the vinegar smell — and cure quickly, but the acid can corrode bare metals, damage the silver backing of mirrors, and stain or etch some natural stones and concrete. Neutral-cure silicones (oxime or alkoxy types) cure without acid, so they are the right choice for mirrors, natural stone, metals, concrete and most facade and window work. Sanitary silicones add a fungicide to slow mould growth in bathrooms and kitchens.

Silicone has two limitations that cause most misuse. First, ordinary silicone cannot be painted: paint beads off it or peels. Second, silicone residue contaminates surfaces, so a joint that once had silicone must be cleaned thoroughly before any other sealant or paint will stick. Silicone also has weaker adhesion to porous, dusty surfaces such as raw concrete and plaster unless a primer is used.

Polyurethane (PU)

PU sealants also cure with moisture, forming a tough, elastic and abrasion-resistant rubber. They bond very well to concrete, masonry, timber and metals, can usually be painted once cured, and resist foot and wheel traffic, which makes them the standard choice for floor joints, expansion joints in concrete, joints between precast panels, and perimeter joints of frames in masonry where the sealant will be painted. Their weaknesses are sunlight and glass: many PU sealants chalk, yellow or harden with long UV exposure, and they are not recommended for bonding or sealing glass in direct sun. PU can also bubble if applied to damp concrete in hot weather, and many grades need a primer on porous surfaces.

Acrylic

Acrylic sealants are water-based: they cure by evaporation of water, shrink slightly, and form a firm, paintable seal. They are easy to apply, smooth with a wet finger or tool, clean up with water and accept paint within hours. That makes them ideal for decorators: filling gaps between skirting and wall, door frames and plaster, cornices and ceilings, gypsum board joints at walls, and hairline corners before painting. Their movement capability is low, and most acrylics are not designed for permanent immersion or exterior movement joints. Used outside or in a wet zone, acrylic cracks, softens or washes out.

Hybrid MS polymer

Hybrid or MS polymer sealants cure by moisture into an elastic rubber and combine several useful properties: good adhesion to many surfaces without primer, including slightly damp ones, UV stability better than most PUs, paintability, and no isocyanates or acid. They are increasingly used for facades, roof details, windows, metal flashings and bonding-and-sealing jobs. They usually cost more than silicone or PU and have somewhat lower movement capability than the best silicones.

How to size a sealant joint

Most sealant failures are joint-design failures, not product failures. A good moving joint has three features. First, the right width: wide enough that the expected movement is within the sealant's movement capability. A 3 mm joint between materials that move 2 mm will tear any sealant; the same movement in a 10 mm joint is easy for a ±25 per cent sealant. Second, the right depth: sealant works best as a thin, hourglass-shaped band that is bonded to the two sides only. As a rule of thumb, the depth of sealant should be about half the width for joints of about 12–25 mm, equal to the width for small joints up to about 10–12 mm, and never less than about 6 mm. Third, no bond to the back: if the sealant sticks to the bottom of the joint as well as the sides — three-sided adhesion — it cannot stretch and will tear.

The coverage column is simple arithmetic: a 300 ml cartridge holds 300,000 mm³ of sealant, so a 6 × 6 mm bead runs about 8 m, a 10 × 10 mm joint about 3 m and a 20 × 10 mm joint about 1.5 m, before wastage. Larger 600 ml sausage packs, used with a sausage gun, are cheaper per millilitre for PU and hybrid sealants on big jobs. Joints wider than about 25–30 mm, such as structural expansion joints between building blocks, usually need a proprietary expansion joint system with covers rather than sealant alone, designed by the structural engineer.

Where each sealant belongs in a home

A typical Indian flat or bungalow uses all three families, and sometimes a hybrid, in specific places. The table below is the practical map. Where a product's datasheet disagrees with the general rule, the datasheet wins.

Bathrooms deserve a special word. Silicone in a shower is a maintenance layer, not the waterproofing. The waterproof membrane under the tiles, the falls to the drain and properly filled grout are what keep water out of the slab; the silicone bead closes the moving joints where tile meets glass, tray, basin or a change of plane, and where rigid grout would crack. When mould grows under a silicone bead, no amount of scrubbing removes it, because it is inside the rubber. The fix is to cut the bead out, clean the joint with a solvent recommended by the manufacturer, let it dry thoroughly — at least a day in a bathroom with the exhaust running — and re-seal with a fresh sanitary silicone.

Windows, doors and the building envelope

The joint between an aluminium or uPVC window frame and the masonry around it is the most important sealant joint in the house, and the one most often filled with cement mortar alone. Aluminium expands roughly twice as much as concrete or brick for the same temperature change, and uPVC more than that, so the gap between frame and wall opens and closes daily. A rigid filler cracks within a season and lets rain in along the frame, which then shows up as damp patches on the inside plaster below the window.

The correct detail separates the two jobs. On the outside, a weather seal of PU, hybrid or neutral-cure weather silicone, 8–12 mm wide and 6–8 mm deep over a backer rod, keeps rain out and takes the movement. If the outside will be painted with the facade, a paintable PU or hybrid is chosen; if the joint is exposed to strong sun and will not be painted, a neutral weather silicone usually lasts longer. On the inside, a paintable acrylic or hybrid closes the gap to the plaster and keeps air and dust out. The space between is packed with foam or a suitable filler, not rigid mortar. Weep holes in the frame — which drain water that gets past the glazing — must never be sealed shut.

Prices and what you get for them

Sealants are sold mainly in 280–300 ml cartridges for a standard caulking gun and in 600 ml foil sausages for larger PU and hybrid jobs. Prices vary widely between general-purpose and professional grades, between clear and coloured, and between brands. The figures below are indicative October 2026 retail ranges in Indian cities, for planning only. Labour for sealant work is usually included in the tiler's, fabricator's or painter's rate; standalone re-sealing is typically quoted per bathroom or per window.

The cartridge price is a small part of the cost of a joint. A ₹300 cartridge that seals a shower for five years is far cheaper than a ₹150 cartridge that turns black in one monsoon and has to be cut out, along with the cost of the tiler's visit. On windows and facades, the cost of access — scaffolding or a rope team on a multi-storey building — dwarfs the price of the sealant, which is why professional-grade products and correct joint design pay for themselves.

Why sealant joints fail

Failures fall into two groups. Adhesive failure is when the sealant peels cleanly off one side of the joint: the surface was dusty, damp, oily, painted with an incompatible paint, contaminated with old silicone, or needed a primer that was not used. Cohesive failure is when the sealant tears through its own body: the joint was too narrow for the movement, too deep, bonded on three sides, or filled with a sealant whose movement capability was too low. The table below links common symptoms to their causes and fixes.

Applying sealant so it lasts

Application is simple but unforgiving. The joint must be clean and dry: old sealant cut out completely, dust brushed and vacuumed out, oil and old silicone removed with a solvent recommended by the sealant maker. A primer is applied where the datasheet asks for one, typically on porous concrete and stone for PU and on some metals for silicone. The backer rod is pressed in to the correct depth, or bond-breaker tape laid in shallow joints. Masking tape on both sides gives a crisp edge. The sealant is gunned in a continuous bead, pushed into the joint rather than laid on top, and tooled within its skinning time — often 10–30 minutes, shorter in Gujarat's summer heat — to press it against both sides and form the slightly concave profile. The tape is pulled immediately after tooling. Cure depth for moisture-curing sealants is typically a few millimetres per day, so the joint should be protected from water and traffic for the time the datasheet states, often 24 hours or more before a shower is used.

Where sealants are heading

Sealants are moving towards lower emissions and longer life. Green building rating systems, including GRIHA and IGBC, encourage low-VOC adhesives and sealants inside homes and offices, and manufacturers are responding with low-odour and solvent-free grades. Hybrid MS polymer sealants are taking over jobs that once needed two products, because one cartridge can be paintable, UV-stable and primer-free. Sanitary silicones with longer-lasting mould resistance, and colour-matched ranges for grouts and stones, are becoming standard. The principles, however, will not change: choose the chemistry for the substrate and the exposure, size the joint for the movement, and never bond on three sides.

Rules for choosing and using sealant

• Use silicone for water, glass, tile and sun where no paint is needed; choose neutral cure for mirrors, stone, metal and facades.
• Use PU for concrete, floor and expansion joints that take traffic or will be painted; keep it off glass in direct sun.
• Use acrylic only for small, dry, interior gaps that will be painted.
• Consider a hybrid MS polymer where one joint must be paintable, UV-stable and bond to mixed substrates.
• Make moving joints at least 6 mm wide, about twice as wide as deep for wider joints, over a closed-cell backer rod.
• Never fill frame-to-wall gaps with rigid cement mortar alone; seal outside and inside.
• Clean, dry and prime as the datasheet says — most failures are adhesion failures.
• Re-seal sanitary joints when mould gets under the bead; do not paint over silicone.

Frequently asked questions

Which is better, silicone or PU sealant?

Neither is better in general; they suit different joints. Silicone is better for wet areas, glass and joints exposed to strong sun that will not be painted. PU is better for concrete, floors, expansion joints and joints that must be painted or will take foot traffic. Using silicone in a floor joint or PU on sunny glazing usually shortens the life of the joint.

Can acrylic sealant be used in a bathroom?

Only for dry areas outside the wet zone, such as the joint between a door frame and plaster that will be painted. Around showers, basins, WCs and bathtubs, use a sanitary silicone, because acrylic is not designed for constant wetting and has low movement capability.

Can you paint over silicone sealant?

Ordinary silicone cannot be painted; paint will bead or peel. Where a joint must be painted, use acrylic for small interior gaps, or PU or a hybrid MS polymer sealant for joints that move more or face the weather.

What is the difference between acetoxy and neutral silicone?

Acetoxy silicone releases acetic acid as it cures, smells of vinegar and cures quickly, but it can corrode bare metals, damage mirror backing and stain some stones. Neutral-cure silicone cures without acid and is the safe choice for mirrors, natural stone, concrete, metals and facades. Both are available in sanitary grades.

How long does sealant take to cure?

Most silicone, PU and hybrid sealants form a skin within about 10–30 minutes and cure inward at a few millimetres per day, depending on temperature, humidity and joint depth. Acrylic dries as water evaporates and can usually be painted within hours. Keep showers dry and floor joints free of traffic for the time stated on the datasheet, often at least 24 hours.