Sika Sikaflex-291
A non-sag, one-component polyurethane adhesive sealant built for the marine trade — elastic, low-odour and forgiving on metal, timber, GRP and painted surfaces.
What is it?
A one-component, moisture-curing polyurethane adhesive sealant from Sika, originally developed for boat and yacht building but equally useful wherever a tough, elastic bond-and-seal is needed on mixed substrates.
Where is it used?
Deck joints, hull-to-deck seams, portlights, fittings and interior sealing on boats; and general elastic jointing on metal, timber and fibreglass assemblies in fabrication and fit-out work.
Why use it?
It sticks to almost everything used in marine construction, stays flexible through -50 to 90 degrees C, resists fresh and salt water, can be over-painted and sanded, and needs no mixing straight from the cartridge.
How is it applied?
Gun-applied from a 300 ml cartridge or 600 ml unipack onto clean, dry, degreased surfaces, tooled within the 60-minute skin time, and left to cure on atmospheric moisture.
About Sika Sikaflex-291
Sikaflex-291 is one of Sika’s most widely specified marine sealants, and for good reason: it does the two jobs a boatbuilder needs most, sealing and bonding, from a single non-sag paste. It is a one-component polyurethane that cures on contact with moisture in the air, so there is no mixing, no pot life to watch and no waste. Once cured it forms a permanently elastic rubber that flexes with the structure, absorbs vibration and holds a watertight seal against fresh water and seawater. Its low odour and non-corrosive chemistry make it comfortable to use in the enclosed spaces typical of hull and cabin work.
For engineers and fabricators in India, the appeal goes beyond boats. The same qualities that suit a yacht deck — reliable adhesion to metals, timber, ceramics, GRP and cured paint systems, plus the ability to be sanded and over-painted — make it a dependable general-purpose elastic sealant for mixed-material assemblies. As an authorised Sika channel, Space Arc supplies Sikaflex-291 across Delhi NCR and Pan-India with the correct primers and activators, so you get a product that behaves exactly as the datasheet promises rather than an unbranded substitute of uncertain shelf life.
The technical insight
The chemistry is what makes Sikaflex-291 so tolerant. Being a one-part moisture-curing polyurethane, it begins to react the moment it meets humidity in the air: a skin forms in about 60 minutes and the material then cures inward at roughly a few millimetres per day, depending on temperature and humidity. Because water content in air falls as temperature drops, cold conditions simply slow the reaction rather than stopping it. The cured film reaches a Shore A hardness of around 40 — soft enough to stay elastic to about 500 percent elongation, firm enough to resist tearing at 6 N/mm. That balance is why it can act as both an adhesive that survives vibration and a sealant that keeps flexing without splitting. Adhesion, though, is entirely dependent on surface preparation and the correct Sika activator or primer for each substrate; the polymer bonds chemically, so a greasy or dusty face will always be the weak link, not the sealant itself.
Where it is used
- Bedding and sealing deck fittings, cleats, rails and hardware on boats and yachts
- Hull-to-deck joints and seam sealing on GRP and metal marine structures
- Sealing portlights, windows and hatches against water ingress
- Bonding and sealing interior panels, trims and linings in cabins and fit-outs
- Elastic, vibration-resistant joint seals between dissimilar metals and timber
- General fabrication jointing on steel, aluminium, ceramics and fibreglass assemblies
- Sealing around fixtures where a paintable, sandable finish is required
- Bonding coated and primed (2-component) painted surfaces that need a flexible seal
Key advantages
- Bonds well to a wide range of substrates — metals, timber, GRP, ceramics and cured paint
- Permanently elastic with about 500 percent elongation, so joints keep moving without cracking
- Single-component and ready to gun straight from the cartridge, with no mixing or waste
- Cures on atmospheric moisture, giving a workable 60-minute skin time for tooling
- Low odour and non-corrosive, suitable for enclosed cabin and interior work
- Can be over-painted and sanded once skinned, for a clean finished appearance
- Wide service range from -50 to 90 degrees C handles marine and outdoor exposure
- Listed under NSF Nonfood Compounds in black and white grades
Limitations & considerations
- Not for permanent underwater immersion or continuously submerged joints — it is a marine sealant, not an immersion-grade tanking product
- Must not be used on stress-crack-prone plastics such as Plexiglas and polycarbonate
- Only temporarily resistant to fuels and mineral oils; not resistant to organic acids, concentrated acids, caustics or solvents
- Adhesion depends entirely on correct cleaning and the right Sika activator or primer for each substrate
- Available only in white and black, limiting colour-matching options
- Cure slows noticeably in cold, dry conditions because it relies on ambient humidity
Selection & best practice
Start with surfaces that are clean, dry and free of grease, oil and dust, then treat each substrate with the Sika activator or primer indicated in the current Sika pre-treatment chart — this step, not the sealant, decides how long the bond lasts. Apply between 5 and 40 degrees C, with 15 to 25 degrees C ideal for both substrate and sealant. Gun the bead, then tool and finish within the 60-minute skin time using Sika Slick or another tested finishing agent. Where the product is used as an adhesive, keep to the recommended bond thickness so the elastic layer can flex. Clean uncured material and tools with Sika Remover-208; once cured it comes off only mechanically. Always run an adhesion trial on the actual substrates before committing to a full run.
Specifications
| Chemical base | One-component polyurethane, moisture-curing |
| Colours | White, black |
| Density (uncured) | 1.3 kg/l |
| Application temperature | 5 to 40 degrees C (optimum 15-25 degrees C) |
| Skin time | 60 minutes at 23 degrees C / 50% r.h. |
| Open time | 45 minutes |
| Shore A hardness | Approx. 40 (ISO 48-4) |
| Elongation at break | Approx. 500% (ISO 527) |
| Service temperature | -50 to 90 degrees C |
Industry use cases
Boat & Yacht Building
Bedding deck hardware, sealing portlights and hull-to-deck seams with an elastic, watertight, over-paintable bond on GRP and metal.
Marine Fabrication & Repair
Vibration-resistant jointing between dissimilar metals and timber on jet’s fittings, workboats and pontoon structures.
Metal & GRP Fabrication
Flexible bond-and-seal for panels, enclosures and assemblies where thermal movement and mixed substrates rule out rigid adhesives.
Interior Fit-Out
Low-odour sealing of cabin linings, trims and fixtures in enclosed spaces, with a sandable, paintable finish once skinned.
Signage & Architectural Metalwork
Elastic sealing of coated metal and composite joints exposed to weather, where movement accommodation and paint compatibility matter.
Frequently asked questions
Can Sikaflex-291 be used underwater or in permanently submerged joints?
What colours and pack sizes does it come in?
Do I need a primer, and how long can I keep the tooling time?
Can it be painted over and sanded?
What substrates does it bond to?
Is Sikaflex-291 the same as Sikaflex-291i?
How we position Sika Sikaflex-291
Sikaflex-291 sits in the marine end of the sealant range — the go-to when you need one paste that both bonds and seals mixed substrates on a boat or fabricated assembly. Where the job calls for a purely interior marine seal, its sister product Sikaflex-291i is the tuned option, while structural marine bonding steps up to Sikaflex-292i. For building and civil movement joints — facades, floors, precast panels — this is the wrong tool; reach instead for a construction-grade PU such as Fosroc Nitoseal PU or a Sika construction sealant, and for sanitary or high-chemical joints a suitable specialised sealant. Space Arc carries all of these brand families, so we can match the exact grade to the exposure rather than forcing a marine product into a civil application.
Fosroc Nitoseal PU — A specifier working on building or civil movement joints — facades, floors, precast — should choose Nitoseal PU, a construction-grade one-part polyurethane, rather than the marine-focused Sikaflex-291.
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Authorised distribution, technical advice & professional application across Delhi NCR and Pan-India.