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Prymax GRP Primer — 1kg & 5kg For GRP Deck Preparation

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About Prymax GRP Primers/Sealers & buying advice

Shop Prymax GRP Primer in 1kg and 5kg quantities for priming OSB3 deck boards before GRP resin application. Prymax GRP Primer is applied to the deck surface before the resin to improve laminate adhesion and seal the deck against resin bleed-through.

Buying guide

Choose Prymax GRP Primers & Sealers for preparing, installing, finishing or maintaining a GRP fibreglass flat roof system.

Good choice when

  • Use this category for preparing, installing, finishing or maintaining a GRP fibreglass flat roof system.
  • Use it when the surface is GRP roofing systems and prepared flat roof details.
  • Use it when the measurements, preparation work and related accessories have been checked.

Consider another route when

  • Do not use this category when you need the main waterproofing system or a complete GRP kit rather than an accessory or component.
  • Do not use it when the roof is unsafe or unsuitable without preparation.
  • Do not use it as a shortcut if another CRS system is a better fit.

Next step: Match the accessory to the roof detail, kit type and installation stage before ordering.

What Happens Without Deck Primer

OSB3 is a porous board material. When roofing resin is applied to an unprimed OSB3 surface, the board fibres absorb a significant proportion of the resin before it can be used to wet out the fibreglass matting. The installer then needs to apply more resin to compensate, which is wasteful, inconsistent and often results in the first mat layer being resin-starved at the board interface even when the surface appears wet. A resin-starved bond to the deck means the laminate can delaminate from the board at that interface over time. Primer eliminates this problem for a small additional cost and a quick application step.

Primer Coverage and Application

Prymax GRP Primer is applied to the OSB3 deck with a brush or short-pile roller and left to become touch-dry before the resin and matting are applied on top. Coverage depends on the board surface condition and absorbency — a 5kg tin typically covers a standard domestic garage or extension roof area. The 1kg tin is sufficient for repair sections, small isolated areas and localised re-priming where a board has been replaced.

Frequently asked questions

What does Prymax GRP Primer do?

Prymax GRP Primer seals the OSB3 deck surface before GRP resin and matting are applied. It prevents the porous board from absorbing resin during lamination, which would otherwise starve the mat of resin and weaken the laminate-to-deck bond.

Is GRP primer the same as the deck primer used for vapour barriers?

No. Prymax GRP Primer is a separate product used to prepare the deck for GRP lamination. Elotene SBS Primer is used to prepare the deck before self-adhesive vapour barriers are laid. They are different products for different stages of the build-up.

How much Prymax GRP Primer do I need?

A 5kg tin is suitable for a standard domestic garage or extension roof. A 1kg tin suits smaller repairs and isolated areas. Check the coverage rate on the product label for the specific deck area being primed.

How long does primer need to dry before resin is applied?

Prymax GRP Primer should be allowed to become touch-dry before resin is applied over it. Applying resin while the primer is still wet dilutes the primer and defeats its purpose. Check the product drying time for the temperature conditions on the day.

Does every GRP roof need primer?

Primer is strongly recommended on new OSB3 deck boards and is standard practice for most GRP roofing installations. It is also used when re-laminating over a previously GRP-laminated surface that has been keyed back. Always check the GRP system guidance for the specific primer requirements.

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