Sikaflex-591 Marine Polyurethane Sealant (White, 300 ml)

Sikaflex-591 is a new-generation marine sealant and adhesive that combines the advantages of polyurethane and silicone technologies in one product. Developed specifically for marine use, it meets the strict demands for strength, durability, and safety in boat construction. Its base is STP – Silane Terminated Polymer, meaning it cures with atmospheric moisture into a tough elastomer without releasing harmful isocyanates.

Key characteristics and benefits:

• Wide application: Sikaflex-591 can be used as a sealant for horizontal and vertical joints (hull-to-deck seams, sealing around deck fittings, hatches, rails, screw holes) and as a light-duty adhesive for bonding components (e.g., attaching trim pieces, small fixtures to the hull or deck). Virtually any sealing job on a boat – from the cabin to the bilge – is within 591’s capability.
• Excellent adhesion without primer: One of its great strengths is its ability to bond well to various substrates even without a primer. Wood, metals (aluminum, steel, stainless), painted or primed surfaces (two-part paints/primers), fiberglass (FRP/gelcoat), ceramic, and most plastics – Sikaflex-591 sticks securely to all of these. This simplifies work and saves time by reducing the need for multiple prep products. (In special cases or permanently submerged joints, Sika does recommend a primer for maximum longevity.)
• Marine-grade durability: Once cured (typically 24 hours for full cure for a 5 mm bead under standard conditions), the product forms a flexible, rubber-like seal that is UV resistant (won’t degrade or yellow from sun), salt-water resistant (ideal for deck and below-waterline applications), and tolerant of temperature swings and mold/mildew. This ensures a long service life of the seal – it won’t crack or shrink over time, maintaining watertight integrity.
• Vibration and sound damping: Thanks to its elasticity (it has a low modulus), Sikaflex-591 absorbs vibrations from engines, waves, and wind, protecting the structure from fatigue. For example, deck hardware bedded with it is less likely to loosen under constant pounding, and interior panels won’t rattle.
• Safe to use and indoor-friendly: It’s free of solvents and isocyanates, meaning low odor and low emissions during curing. This is crucial when working in enclosed areas (cabins, yachts) – there are no harsh fumes. It’s also halogen-free and neutral-curing, so it won’t corrode metals (no acidic byproducts like some silicones). It carries certifications for flame retardancy (IMO FTP) and low VOC, allowing use even in living quarters on ships.

How to use:

1. Surface prep: Surfaces should be clean, dry, and dust-free. Remove old paint, rust, or sealant residues. Degrease with Sika Aktivator or isopropanol especially if oils are present. (Follow Sika’s guide – for some metals a primer might be advised, though 591 generally bonds well without one.)
2. Application: Sikaflex-591 comes in a 300 ml cartridge for standard caulk guns. Cut the tip and attach the nozzle, trimming it at a 45° angle to the desired bead size. Apply with steady pressure – it extrudes as a thick paste. It can be applied in temperatures from +5°C to +40°C, but ideally between 15-25°C.
3. Tooling: Skin-over time is about 30 minutes (at 23°C/50% humidity). Before it skins, you can tool the joint with a spatula or a finger dipped in soapy water to achieve a smooth finish. Remove excess sealant promptly – once cured, removal is more difficult (mechanical).
4. Curing: It cures roughly 2-3 mm in 24h (depending on humidity and temperature). Avoid moving bonded parts or subjecting the seal to water until the sealant has set sufficiently (e.g., avoid heavy rain in the first 6-8 hours).
5. Cleanup: Fresh, uncured sealant can be cleaned off with Sika Remover-208 or acetone on a cloth. Cured material can only be removed mechanically.

Tips: For teak deck caulking, use the dedicated Sikaflex-290 DC (591 is multi-purpose, but for teak decks 290 DC is optimized). Do not use 591 on stressed transparent plastics like PMMA/polycarbonate windows – for those, Sikaflex-295 UV is designed.

Real-world use: Boaters appreciate Sikaflex-591 for its reliability and versatility. For instance, when mounting new cleats or hardware on deck, 591 creates a waterproof seal beneath the fitting that won’t leak even under shock loads. Inside the cabin, around window frames or between interior modules, 591 eliminates vibration noise and ensures that in rain, water stays outside. Many technicians report that this product has effectively replaced the traditional Sikaflex-291 in their daily work – it offers the same (or better) adhesion and flexibility, but with milder odor and a better health profile. In short, Sikaflex-591 is the “Swiss Army knife” of marine sealants – whatever sealing job you have on board, it’s likely the right tool for it.