G4 Black Pond Sealer

G4 Black Pond Sealer

Black moisture-cured polyurethane pond sealer for rendered concrete ponds

G4 Black Pond Sealer is a single pack moisture-cured polyurethane pond sealer designed to form a non-porous waterproof coating on suitable rendered concrete, brick and porous stone surfaces. It is widely used for koi pond construction and pond refurbishment because it bonds into porous surfaces and cures to form a tough protective film.

G4 is different from ordinary decorative paint because it cures through reaction with moisture in the air and within the substrate. This makes it well suited to pond sealing work where the surface is slightly damp but still porous enough for the first coat to penetrate. Correct penetration is important because the first coat creates the mechanical bond that allows the coating system to perform properly.

The black version provides a black pigmented finish for a deeper, cleaner pond appearance. Once fully cured and washed down, G4 creates a hard waterproof barrier that helps prevent water loss through porous render and concrete. It is suitable for many pond builds, but the structure underneath must still be sound. G4 will not correct movement, hollow render, failed blockwork or serious cracking.

New concrete or render needs proper curing before sealing. As a general rule, new render should be kept damp during its early cure and then allowed to cure before the coating system is applied. Waterproofing additives should not be used in the render mix because they can reduce porosity and stop the first coat from bonding properly. If a waterproofing additive has already been used, the surface may need abrading to create a suitable key.

For black G4 systems, clear coats are normally used first to achieve the best penetration into porous render, followed by pigmented colour coats. If you are coating a new pond in black, compare this page with G4 Pond Sealer Clear so the full coating system is planned correctly.

When to choose G4 Pond Sealer

Choose G4 when you want a proven moisture-cured polyurethane system for rendered concrete ponds and water features. It is particularly useful where a coating needs to bond into a porous surface and where the pond structure is already correctly built, rendered and ready for sealing.

• Ideal for rendered concrete koi ponds and formal ponds
• Useful for sealing porous concrete, brick and stone surfaces where suitable
• Can be applied to slightly damp porous surfaces when conditions are suitable
• Suitable for cold and humid conditions compared with many coating systems
• Not a structural repair for movement, hollow render or major cracks

Application and preparation guidance

Apply G4 by brush, working the first coat well into the surface. Rollers can be used on larger vertical areas, but care is needed to avoid puddling on the base. Subsequent coats should be applied while the previous coat remains finger-tacky, similar to the feel of adhesive tape. Do not allow more than the recommended maximum time between coats.

Surfaces must be clean, stable and free from dust, grease, algae, loose render and contamination. Any exposed reinforcing fibres should be removed before coating because the sealer will not properly bond to protruding fibres. After full chemical curing, wash the surface thoroughly, drain away the wash water and then refill.

Key benefits

• Single pack moisture-cured polyurethane pond sealer
• Forms a non-porous waterproof seal on suitable prepared surfaces
• Excellent penetration into porous render and concrete
• Can be applied in cold or humid conditions when used correctly
• Long lasting chemically resistant finish once fully cured
• Available in clear and black options
• Popular for koi pond construction, refurbishment and formal pond sealing
• Clear technical application process for professional pond builders

Available sizes

• 1kg tin
• 2.5kg tin
• 5kg tin
• 25kg tin

Technical specifications

• Product type: Single pack moisture-curing polyurethane pond sealer
• Brand: G4
• Colour: Black
• Suitable surfaces: Rendered concrete, brick and porous stone where properly prepared
• Intended use: Pond sealing and waterproofing
• Finish: Non-porous waterproof coating after full cure
• Coverage guidance: G4 Clear three coats approximately 550g per square metre
• Colour system guidance: Colour systems normally use clear coats first, followed by pigmented coats
• Mechanical cure: Approximately 24 hours
• Full chemical cure: Approximately 72 hours
• Minimum application temperature: Down to 0°C when conditions are otherwise suitable
• Application method: Brush recommended; roller may be used carefully on walls
• Before final fill: Wash, drain and refill only after full cure

Related pond paint products

Use these links to compare G4 with the other pond coating options before choosing the right system for your pond.

• G4 Pond Sealer Clear
• Gold Label Pond Paint Black
• Gold Label Pond Paint Clear
• Prymax Pond Sealer – Clear
• Prymax Pond Sealer – Black
• Browse all pond paint and pond sealer products

Important note before filling the pond

Allow the coating to cure fully before filling. Once cured, wash the surface, drain away the wash water and refill with fresh dechlorinated water. Fish should only be returned once the pond has been rinsed, refilled and checked for suitable water quality.