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G4 Damp Seal

G4 Damp Seal

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  1. 1kg G4 Sealer
    £28.08 £23.40
  2. 2.5kg G4 Sealer
    £54.00 £45.00
  3. 5kg G4 Sealer
    £74.81 £62.34
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G4 Sealer, also referred to as G4 Damp Seal is a moisture curing polyurethane liquid that forms a non-porous coating on concrete, brick or porous stone. G4 utilizes the moisture in the air and substrate as a catalyst to cure and harden.

G4 Sealer, also referred to as G4 Damp Seal is a brown, translucent, polyurethane which hardens when in contact with moisture. Once applied, it will provide a tough damp-proof seal for most porous and non-porous substrates such as concrete, brick, stone or wood.

Application of G4 Damp Seal

G4 can be applied with a brush or roller. It can be applied to substrates which contain moisture, although it is important to note that the area should be dry enough to be porous. If the area is too damp, this can cause the G4 to de-laminate or blister. Once the area is dry enough to be porous, this will ensure that the first application of the G4 sealer will adhere to the surface area. G4 should not be used in temperatures below zero degrees.

Ideally, the area should be free from all existing coatings and wiped down with Acetone before application. However, if this is not possible the adhesion will depend on the existing coating. If applying to brick, any loose pointing should be removed before G4 is applied.

It is also important to note that G4 is not effective in areas of direct UV light; therefore if UV resistance is required a topcoat should be used to enhance UV protection.

After the G4 is applied, we recommend that brushes are cleaned immediately with Acetone and then dried.

Uses of G4 Damp Seal

G4 Damp Seal can be used for coating walls, floors and as a pre-coat for fibreglass roof builds and ponds.


In the case of fibreglass pond building, G4 Damp Seal helps to prevent lime entering into the water which is extremely important to the health of any fish in the pond.

Thanks to its polyurethane base, G4 can be applied directly to the concrete and will cure in approximately 30-60 minutes, depending on the humidity. It should be brushed on with a coverage of 250g/m2.


Most fibreglass roofers will ensure G4 Damp Seal is used as a bonding primer between the substrate (concrete, plywood, metal or OSB Boards) and the lamination (catalysed resin and chopped strand matting)

G4 Damp Seal should be applied to a clean substrate which is free from contamination; this often requires the area to be wiped down with Acetone prior to applying G4.

Wood sealing

An effective use of G4 Sealer can be to varnish wood for internal use. It can be used even in cases of low temperatures or high levels of humidity. It is best not to use G4 for varnishing wood, which will be kept outside as it can be affected by UV light. However, G4 does have an excellent reputation as a varnish, and in terms of performance, it is rated as one of the best on the market.

Where to buy G4 Damp Seal

FibreGlassDirect is a proud supplier of G4 Damp Seal. We are located in Gloucestershire and Newry, Northern Ireland and therefore in a position to distribute G4 quickly to anywhere in the UK and Ireland. Our G4 Damp Seal is available in 1kg2.5kg and 5kg tin sizes.

We recommend 1 layer of G4 as a primer on concrete and brick to promote adhesion between the Fibreglass lining and the concrete surface. Application rate is 250gm/m2.

 G4 Damp Seal

Warning! Persons already sensitised to diisocyanates may develop allergic reactions when using this product. Persons suffering from asthma, eczema or skin problems should avoid contact, including dermal contact, with this product. This product should not be used under conditions of poor ventilation unless a protective mask with an appropriate gas filter (i.e. type A1 according to standard EN 14387) is used.’ Protective gloves to be supplied with this product.

Please be aware that from 24th August 2023, all professional and industrial workers (employed or self-employed), using mixtures containing diisocyanates at a concentration of 0.1% or more (e.g., insulation and filler foams used in construction etc.), must undergo mandatory training.
  Diisocyanate QR Code

Hazard Statement - Bonda G4 Damp Seal Danger H226 - Flammable liquid and vapour.
H315 - Causes skin irritation.
H317 - May cause an allergic skin reaction.
H319 - Causes serious eye irritation.
H332 - Harmful if inhaled.
H334 - May cause allergy or asthma symptoms or breathing difficulties if inhaled.
H335 - May cause respiratory irritation.
H336 - May cause drowsiness or dizziness.
H351 - Suspected of causing cancer (if swallowed, if inhaled).
H373 - May cause damage to organs through prolonged or repeated exposure (if swallowed, if inhaled).
H411 - Toxic to aquatic life with long-lasting effects.
Health & Safety Warning Labels