Hempel Underwater Primer

Hempel’s Underwater Primer 26030 is a fast-drying, aluminium-flake primer used as a sealer over sound old antifouling or as a tiecoat between an epoxy primer and antifouling. It is suitable for glass fibre, wood and correctly primed steel below the waterline, including keels, with a current theoretical coverage of 7.8m²/L.

Designed for the preparation and maintenance of underwater hull surfaces, Hempel’s Underwater Primer creates a compatible intermediate layer before the application of Hempel antifouling. It can be used to seal an existing antifouling coating after suitable abrasion and cleaning, or applied as a tiecoat over an epoxy-primed surface.

The required coating system depends on the substrate. Apply one coat as a tiecoat over an epoxy primer while the epoxy surface is almost dry but still has a slight tack. On previously coated surfaces, apply one full undiluted coat after abrasion, cleaning and drying. Porous surfaces require an initial coat thinned by up to 20%, followed by two full undiluted coats.

Hempel’s Underwater Primer is suitable for glass fibre, wood and steel in below-waterline systems. New wood should first receive a suitable wood treatment, with Hempel’s Sealer listed as the recommended preceding product. Steel must be cleaned, freed from rust and loose material, and coated with the specified Hempel primer before Underwater Primer is applied.

Coverage note: Hempel’s current December 2025 Product Data Sheet specifies a theoretical spreading rate of 7.8m²/L at a dry film thickness of 50µm.

Intended Applications and Compatibility

• Suitable for sealing sound, previously applied antifouling
• Suitable as a tiecoat between an epoxy primer and antifouling
• Suitable for preparing underwater hull surfaces for Hempel antifouling
• Suitable for glass fibre and GRP hulls
• Suitable for wood within the specified coating system
• Suitable for correctly prepared and primed steel
• For use below the waterline
• Suitable for keels and other submerged areas
• Compatible with Hempel’s Sealer on new wood
• Compatible with Hempel’s Light Primer
• Intended for overcoating with a suitable Hempel antifouling

Substrate note: Aluminium is not included in the approved substrate list in the current Product Data Sheet. Use a Hempel coating system specifically approved for aluminium.

Surface Preparation and Application Guidance

• Remove oil, grease and other contaminants with a suitable detergent cleaner.
• Clean the surface with Hempel’s Pre-Clean where appropriate.
• Sand suitable uncoated surfaces and remove all dust.
• On new wood, saturate the clean, dry surface with a suitable wood treatment and allow it to dry.
• On steel, remove salts and other contaminants by high-pressure fresh water cleaning.
• Remove rust and loose material from steel by abrasive blasting or suitable power-tool cleaning.
• Remove all residues and apply the specified Hempel primer before applying Underwater Primer.
• On previously coated surfaces, abrade, clean and dry the coating.
• Spot-repair damaged areas and sand to obtain a uniform surface profile.
• Stir Hempel’s Underwater Primer thoroughly before use.
• Apply by brush, roller or paint pad.
• Keep thinning to the minimum required and do not exceed 5% for manual application.
• For porous surfaces, apply the first coat thinned by up to 20% with Hempel’s Thinner 811 08111.
• Follow the first thinned coat on porous surfaces with two full undiluted coats.
• Over an epoxy-primed surface, apply one coat as a tiecoat.
• Apply the tiecoat while the epoxy surface is almost dry but still has a slight tack.
• On a previously coated surface, apply one full undiluted coat.
• Overcoat with the selected Hempel antifouling.
• Airless spray application is possible with up to 20% thinning; contact Hempel for spray settings.
• Apply to a clean, dry surface at least 3°C above the dew point.
• Keep relative humidity below 85% during drying and curing.
• Maintain adequate ventilation during application, drying and curing.
• Avoid condensation on the freshly applied coating.
• Use Hempel’s Thinner 811 08111 to clean compatible application equipment.

Key Features

• Hempel product code 26030
• Fast-drying underwater primer
• Contains aluminium flakes
• Seals sound old antifouling
• Functions as a tiecoat over epoxy primer
• Intended for overcoating with Hempel antifouling
• Suitable for glass fibre, wood and correctly primed steel
• For use below the waterline, including keels
• Standard aluminium-grey shade 19000
• Theoretical spreading rate of 7.8m²/L
• Reference dry film thickness of 50µm
• Reference wet film thickness of 129µm
• Volume solids of 39 ± 2%
• Specific gravity of 1.1kg/L
• Apply by brush, roller, paint pad or airless spray
• Hempel’s Thinner 811 08111 specified for thinning and tool cleaning
• Maximum manual-application thinning of 5%
• Maximum airless-spray thinning of 20%
• Maximum first-coat thinning of 20% on porous surfaces
• Touch dry in approximately three hours at 20°C
• Touch dry in approximately six hours at 10°C
• Minimum overcoating interval of 2½ hours at 20°C
• Minimum overcoating interval of 4½ hours at 10°C
• No fixed maximum overcoating interval, subject to surface condition

Frequently Asked Questions

What is Hempel’s Underwater Primer 26030 used for?
It is used as a sealer over sound old antifouling and as a tiecoat between an epoxy-primed surface and antifouling.

Is Hempel’s Underwater Primer itself an antifouling?
No. It is a primer, sealer and tiecoat. It must be overcoated with a suitable Hempel antifouling to provide fouling protection.

Can it be applied over old antifouling?
Yes, provided the existing antifouling is sound. Abrade, clean and dry the surface before applying one full undiluted coat.

Can it be applied over an epoxy primer?
Yes. Apply one coat as a tiecoat while the epoxy primer is almost dry but still has a slight tack.

Can it be used on GRP or glass fibre?
Yes. Sand suitable uncoated surfaces and remove all dust before application.

Can it be applied directly to steel?
The current Product Data Sheet requires steel to be cleaned, freed from rust and loose material, and primed with the specified Hempel primer before Underwater Primer is applied.

Can it be used on new wood?
Yes. New wood must be clean and dry, treated with a suitable wood treatment and allowed to dry. Hempel’s Sealer is the recommended preceding product.

Can it be used on keels?
Yes. It is specified for all below-waterline areas, including keels. Follow the correct preparation and primer system for the keel material.

Can it be used on aluminium?
Aluminium is not included in the approved substrate list in the current Product Data Sheet. Use a Hempel system specifically approved for aluminium.

How many coats are required?
Apply one tiecoat over epoxy primer, one full undiluted coat over a suitable previously coated surface, or an initial thinned coat followed by two full undiluted coats on porous surfaces.

What is the coverage?
The current theoretical spreading rate is 7.8m²/L at a dry film thickness of 50µm.

Which thinner should be used?
Use Hempel’s Thinner 811 08111. The same thinner is specified for cleaning application equipment.

How much thinner can be added?
Maximum thinning is 5% for brush, roller and paint-pad application. Airless spray and the first coat on porous surfaces may be thinned by up to 20%.

How long does it take to dry?
It is touch dry in approximately three hours at 20°C or six hours at 10°C.

When can it be overcoated?
The minimum stated interval is 2½ hours at 20°C or 4½ hours at 10°C. Clean and lightly abrade contaminated or prolonged-exposure surfaces before recoating.

Can it be used above the waterline?
Hempel specifies Underwater Primer 26030 for areas below the waterline, including keels.