Bostik Seal 'N' Flex One P390 Polyurethane Sealant 600ml

Bostik Seal 'N' Flex One P390 is a low-modulus polyurethane sealant for expansion and construction joints in precast concrete, tilt-up panels, brickwork and fibre cement cladding. This 600ml sausage is suited to installers sealing long runs of moving joints, with a flexible, paintable finish and strong adhesion to properly prepared building substrates.

How it works

Seal 'N' Flex One P390 is a one-component, moisture-curing polyurethane paste that cures to form a tough, flexible seal. Because it is a low-modulus sealant (Shore A hardness of 30), it exerts minimal stress on the bond line when the joint expands or contracts. This properties profile enables the sealant to handle cyclic expansion and compression movement up to ±50% of the original installed joint width. It does not sag during application and cures at a rate of 2mm per 24 hours on porous substrates, making it suitable for vertical expansion joints in external facades.

Numbers that matter

• ±50% joint movement: Absorbs significant thermal expansion and contraction across precast panels, brickwork, and window frames without adhesive failure.
• > 900% elongation at break: High elasticity to stretch across widening joints under extreme thermal cycles before structural failure.
• 6 to 12 hours tack-free time: Provides a working window to tool the joint while resisting early dust pick-up on site.
• 7 to 14 days full cure: Reaches full cure in 7 days on porous substrates, extending to 14 days for deep joints over 20mm wide.
• 6mm to 50mm joint limits: Accommodates joint depths from a minimum of 6mm up to a maximum width of 50mm, using a depth-to-width ratio of 1:1 for joints up to 12mm and 1:2 for joints between 12mm and 50mm.

Where to use it

This sealant adheres to a wide range of common construction materials. It is suitable for:

• Insitu concrete, render, precast, and tilt-up panels
• Brickwork, blockwork, and fibre cement sheet
• Plasterboard face and paper-lined recessed edges
• Aluminium windows, GRC, fibreglass, and specialty panel systems
• u-PVC outlets, brass, copper fittings, and stainless steel trays

System priming and preparation

While Seal 'N' Flex One P390 adheres to standard dry concrete, mortar, and bare Aluminium without a primer, specific applications require targeted preparation to prevent bond failure:

• Porous substrates: For optimum performance in all periodically immersed, submerged, or high-stress applications, prime porous joint faces with Bostik N49 Primer.
• Non-porous substrates: For plastics and metals (including u-PVC, brass, copper, and stainless steel), prime with Bostik N40 Primer using the two-cloth method before applying the sealant.
• Joint backing: Always install a backing rod before priming to control sealant depth and ensure a three-sided bond is avoided. Tool the sealant to a concave profile to distribute stress evenly.

Limitations

• Do not use in chlorinated water such as swimming pools or spas, or in constantly immersed salt water.
• Do not apply to cement-based substrates within 28 days of the initial pour or set.
• Do not use on any materials containing bitumen, or for glazing applications.
• Not recommended for trafficable joints.
• Do not apply in temperatures below 5°C or above 35°C.
• Avoid exposure to water or alcohol before the sealant has completely cured.
• Do not finish the joint using wet tooling techniques that involve solvents, water, or detergent/soap solutions.
• Do not apply to joints less than 5mm wide and 6mm deep, or larger than 50mm wide.
• White sealant may yellow on the surface under prolonged exposure to strong natural or artificial UV light.
• For composite metallic façade systems subject to rapid daily thermal movement, early joint movement before full cure may cause permanent crimping or bulging. Bostik V70 Silicone is recommended for these high-movement metal façades.
• If painting, wait a minimum of 7 days. Solvent-based coatings (such as enamels) may react with the sealant to create a tacky surface, and paint films may crack or craze during joint movement. Always conduct a field test first.