Antifouling & Hull Painting Guide

Most antifouling problems – paint lifting, flaking or fouling early – come from preparation, not from the paint. This guide covers the two situations you'll meet: a new or bare hull, and a hull that already has antifouling on it.

Before you start

• Know what's on the hull. The right primer depends on the existing coating. If you don't know what antifouling was used before, treat it as unknown (see antifouling compatibility).
• Know the hull material. GRP, wood, steel and aluminium need different primers, and aluminium must never have a copper-based antifouling.
• Check the weather. Paint needs dry conditions and the right temperature to cure. Avoid painting in direct midday sun in summer, in rain, or when dew will settle on wet paint.
• Work out the quantity. Use our antifouling calculator.

Painting over existing antifouling

This is the usual annual job.

1. Wash straight after haul-out. Pressure-wash the hull before growth dries hard on it.
2. Inspect. Look for flaking or lifting paint, blisters, damage and worn anodes.
3. Sand. Sand the old antifouling to remove loose material and give the new paint a key. Use wet sanding where possible to keep dust down – antifouling dust is toxic.
4. Remove loose paint. Anything flaking or lifting must come off. Feather the edges of bare patches.
5. Prime bare patches. Prime any areas sanded back to the hull, using the right primer for the material.
6. Seal if needed. If the old antifouling is unknown or incompatible, apply a sealer such as Seajet 015 Underwater Primer over it.
7. Apply two coats. Two full coats, plus an extra coat on the waterline, leading edges, keel and rudder, where wear is highest.

Painting a new or bare hull

1. Degrease. New GRP has wax and release agents on the surface that stop paint sticking. Degrease thoroughly before sanding.
2. Sand. Sand the whole underwater hull to a uniform matt finish, then remove all dust.
3. Prime. Build up an epoxy primer system such as Seajet 117. Four coats also protects GRP against osmosis.
4. Apply antifouling within the overcoating window the primer's data sheet gives, so the antifouling bonds to the primer.
5. Two full coats of antifouling, plus extra on high-wear areas.

Coats and thickness

The film thickness is what makes antifouling last. Thin coats wear through early. Two full coats is standard for a season in Maltese waters. Self-polishing antifoulings wear away gradually by design, so the thickness you apply is how long it lasts.

Drying and relaunch

Each coat needs to dry before the next, and the final coat needs time to dry before the boat goes back in the water. The drying times depend on the product and the temperature – the product's data sheet gives the minimum and maximum times. Don't launch earlier than the data sheet allows, and don't leave some antifoulings too long before launch either: check the maximum time out of the water on the data sheet.

Propellers and running gear

Normal antifouling doesn't stay on propellers. Use a dedicated propeller coating such as Seajet Pellerclean, applied to clean, degreased metal.

Safety

Antifouling contains biocides. Wear gloves, eye protection and a mask when sanding and painting, and don't let dust or washings run into the sea. See safety.

Prefer us to do it?

We antifoul boats at yards all over Malta and Gozo. See antifouling application.