Bostik Seal 'N' Flex One P390 Polyurethane Sealant 300ml

For expansion joints in precast and tilt-up panels, brickwork, concrete, and fibre cement sheet, Seal 'N' Flex One P390 Polyurethane Sealant 300ml provides a Class-A joint seal that accommodates high-movement thermal cycles. It cures to a tough, flexible barrier that handles constant contraction and expansion without adhesive failure or pulling away from the joint faces.

High-movement joint sealing

Seal 'N' Flex One P390 is a low-modulus, moisture-curing polyurethane sealant formulated to handle cyclic joint movement of ±50% (100% total movement) of the original installed joint width. Because it is low modulus, the cured sealant exerts minimal stress on the bond line when the joint expands, preventing cohesive or adhesive failure in porous substrates like insitu concrete, mortar, and brickwork. The smooth, non-sag thixotropic paste holds its shape during application in vertical joints and can be tooled to a concave finish. It is acoustic rated to AS 2253 / 1276 and carries AS4020 approval for potable water contact.

Numbers that matter

• ±50% joint movement: Absorbs high-amplitude expansion and contraction in precast panels, GRC, and specialty exterior cladding panels without rupturing.
• > 900% elongation at break: Extreme stretch capability to ensure the internal body of the sealant does not tear under unexpected structural shift.
• > 1.3 N/mm² tensile strength: Delivers high structural cohesive strength to maintain a durable barrier over a service temperature range of -40 to 75°C.
• 7-day full cure: Based on porous substrates, extending to 14 days for joints wider than 20mm, with a cure rate of 2mm per 24 hours.
• 6mm to 50mm joint limits: Fits joint widths within this range, maintaining a 1:1 depth-to-width ratio up to 12mm wide, and a 1:2 ratio for joints from 12mm to 50mm wide.

Where to use it

Suitable for sealing joints in:

• Precast and tilt-up concrete panels
• Insitu concrete and render
• Brickwork and blockwork
• Fibre cement sheet and GRC
• Plasterboard face and paper-lined recessed edges
• Aluminium windows, brass, copper, stainless steel, and u-PVC
• Fibreglass and specialty panel systems

Limitations

• Do not use in chlorinated water systems such as swimming pools, spas, or constantly immersed salt water.
• Do not apply to cement-based substrates within 28 days of the initial pour or set.
• Do not use on materials containing bitumen, for glazing applications, or in trafficable joints.
• Do not apply in temperatures below 5°C or above 35°C, or expose to water or alcohol before full cure.
• Do not finish using wet tooling techniques that employ solvents, water, or soap solutions.
• White variant caution: White sealant may yellow on the surface under prolonged exposure to strong natural or artificial UV light.
• Composite metallic facades: For northern and western elevations in full sunlight, daily thermal cycling can cause crimping or bulging of the sealant before it fully cures. Refer to Bostik V70 Silicone as the recommended sealant for metallic facades.

Substrate preparation and priming

Surfaces must be completely clean, dry, and free of laitance, oil, dust, grease, frost, water, old sealants, and curing agents. Dust and loose particles must be vacuum cleaned.

Seal 'N' Flex One P390 adheres without primer to standard concrete, mortar, and bare Aluminium. However, a Pre-tested Adhesion to Substrate (P.A.T.S.) test is recommended prior to application. Substrate primers must be used for high-stress joints, extreme weather exposure, or highly porous surfaces:

• Porous substrates: For maximum performance and in periodically immersed or submerged applications, prime with Bostik N49 Primer. Porous vertical expansion joints not subject to immersion do not require priming if clean and dry.
• Non-porous substrates: Prime plastics and metallic substrates (such as u-PVC outlets, brass, copper fittings, and stainless steel) with Bostik N40 Primer using the two-cloth method.

Paintability

Seal 'N' Flex One P390 can be painted after a minimum of 7 days. Coatings containing solvents, such as enamels or oil-based paints, may react with the sealant surface and create a tacky finish. Surface coatings may discolour in direct contact with the cured sealant, and cyclical joint movement may cause the paint film to crack or craze. Field testing is recommended to verify coating compatibility.