In short: Choose antifouling for how you use the boat, where you sail and what the hull is made of.
- Self-polishing antifouling suits most sailing and motor boats at normal speeds. It wears away evenly and does not build up thick layers.
- Hard antifouling suits fast motor boats and hulls that are brushed or scrubbed.
- An aluminium hull, sterndrive or saildrive needs a copper-free antifouling made for aluminium.
- In Denmark, use only products approved for use in Denmark that private users may apply, and follow the data sheet.
For most leisure boats in Danish and Baltic waters, a self-polishing antifouling approved in Denmark is the right starting point. Fast planing boats need a hard antifouling, and anything aluminium needs a copper-free one. Antifouling keeps weed, barnacles and mussels off the hull; even a thin layer of fouling slows the boat, raises fuel use and dulls a yacht's sailing performance. The paint's technical data sheet always has the final word on primer, number of coats, drying times and launching.
How does antifouling work?
Antifouling contains biocides, usually copper compounds, that leach slowly from the surface and make it hard for fouling to settle. When the active ingredient is used up, the protection stops. That is why most antifoulings must be renewed every year before launching. Some products are made to last more than one season; the data sheet says so.
Hard or self-polishing antifouling?
For most leisure boats at normal speeds, self-polishing antifouling is the easier choice. Hard antifouling is for fast boats and for hulls that need scrubbing.
| Property | Hard antifouling | Self-polishing antifouling |
|---|---|---|
| How it works | Forms a firm film that stays in place while the biocide leaches out | Wears away slowly in the water, constantly exposing fresh biocide |
| Suits | Fast, planing motor boats and hulls that are brushed or scrubbed | Most sailing and motor boats at normal speeds |
| Advantages | Tolerates high speed and mechanical cleaning | Does not build up thick layers and needs less sanding when repainting |
| Drawbacks | Layers build up over the years and must eventually be removed | Does not tolerate scrubbing, and the film gets thinner over the season |
Whatever the type, the label and data sheet must cover the water you sail in.
Does antifouling work differently in the Baltic?
The paint works the same way, but the fouling changes with salinity, and a boat in Danish waters can cross the whole range in one season. Salinity falls from around 30–35 psu off Jutland's west coast to about 7–8 psu around Bornholm, with the Kattegat, the Belts and the Sound in between. Lower salinity still brings hard fouling. The bay barnacle (Amphibalanus improvisus), a common fouler of hulls and water intakes, tolerates salinity from under 1 to 35 psu, and Baltic populations appear adapted to low salt. In practice:
- Choose a product whose label covers the water the boat lies in for most of the season.
- Moving between the North Sea and the Baltic? Choose for where the boat spends longest, and check that the label covers both.
- A boat that lies mainly in fresh water must not have biocidal antifouling in Denmark; see the rules below.
Which antifouling for aluminium hulls, drives and propellers?
If the hull, sterndrive or saildrive is aluminium, do not use antifouling that contains copper. Copper and aluminium cause galvanic corrosion that can eat into the metal. Use a copper-free antifouling made for aluminium, with the primer the manufacturer specifies.
- Propeller and shaft: do not use ordinary antifouling unless the data sheet allows it. There are special products for propellers.
- Echo sounder and log transducers: some must not be painted at all, others only with a particular type of paint. Follow the transducer maker's instructions.
- Anodes: never paint them. Read why in Zinc, aluminium or magnesium anodes?
What are the rules for antifouling in Denmark?
Antifouling is a biocidal product, so its sale and use are regulated, and the Danish authority is Miljøstyrelsen, the Danish Environmental Protection Agency. In practice:
- Use only antifouling approved for use in Denmark. Approval is national, so a paint you can buy at home in Germany, the Netherlands, Sweden or the UK is not automatically approved here. Miljøstyrelsen publishes an overview of the antifoulings that private users may legally apply.
- Some products are for professional use only. The label says who may use the product.
- Miljøstyrelsen's overview states that biocidal antifouling must not be used on leisure boats that are mainly used in fresh water.
- Collect sanding dust, scrapings and paint residue, for example on a tarpaulin under the boat, and dispose of it as hazardous waste as the harbour or local council directs. Old layers can contain substances that are now banned.
- Many harbours have their own rules for washing, sanding and painting on the hard.
The rules change. Check the current rules with Miljøstyrelsen and ask the harbour before you start. If you also keep the boat in Sweden or Germany, those countries run their own approvals.
Can you change antifouling, and do you need a primer?
Yes, as long as the new paint can bond to the old. As a rule:
- Same type from the same maker: washing and a light sanding are often enough.
- Self-polishing over hard: often possible if the hard paint is sound and sanded.
- Hard over self-polishing: normally means removing the old paint or sealing it with a primer.
- Unknown old paint: use the primer the manufacturer recommends as a barrier coat, or sand back to a sound surface.
- Bare fibreglass, wood or metal: needs preparation and primer from the manufacturer's paint system.
Stick to one paint system, meaning primer and antifouling from the same manufacturer. Our antifouling range includes International, Jotun and Seajet, and each describes its system in the data sheets.
How do you work out the hull area and paint quantity?
Multiply the underwater area by the number of coats and divide by the coverage in square metres per litre from the data sheet. The calculator further down this page does the sum and adds 10 per cent for waste.
If you do not know the area, the paint maker International estimates it as waterline length × (beam + draught) × a hull factor: 1.0 for full-bodied craft, 0.75 for medium-draught racing craft and 0.5 for fin-keeled racing craft. With figures for illustration only: a fin-keeled boat with an 8 m waterline, 3 m beam and 1.5 m draught gives 8 × (3 + 1.5) × 0.5 = 18 m². Two coats at 10 m² per litre need 18 × 2 ÷ 10 = 3.6 litres, before extra for the waterline and waste.
As a rule of thumb, apply two coats on a new or stripped surface, plus an extra coat along the waterline and on the leading edges of keel and rudder, which wear faster than the rest of the hull. Follow the data sheet.
How do you antifoul a boat, step by step?
- Pressure-wash the hull as soon as the boat comes out in autumn, while the fouling is soft. Use the harbour's wash-down area if it has one.
- Scrape off loose flakes and sand lightly, preferably wet, so no dust forms. Wear a respirator, gloves and goggles. Abrasives and accessories are under sanding and polishing.
- Rinse with fresh water and let the hull dry completely.
- Mask the waterline, and tape over anodes and transducers.
- Apply primer if the data sheet or a change of paint requires it.
- Stir the paint thoroughly. The copper settles at the bottom of the tin.
- Apply the first coat. Do not paint in rain, dew or cold; the data sheet gives the minimum temperature.
- Wait the time the data sheet states, then apply the second coat and the extra coat on exposed areas.
- Remove the tape before the paint has fully cured, for a clean edge.
- Keep within the minimum and maximum times before launching. Agree with the yard how to paint the patches under the stands and keel, for example while the boat hangs in the slings.
Never move several boat stands at once yourself to paint under them.
What are the common antifouling mistakes?
- Coats that are too thin, especially at the waterline and on leading edges.
- Too short or too long between coats.
- New paint over loose old paint, so the lot flakes off.
- Copper antifouling on aluminium.
- Paint on anodes and transducers.
How much antifouling do you need?
Enter the bottom area and the coverage from the paint's data sheet (m² per litre per coat).
Includes 10 % for waste. Don't know the area? Most paint makers have an area calculator on their website.
Frequently asked questions
How often should I antifoul?
Most antifoulings need renewing every year before launching; some are made for several seasons. Look at the hull when the boat comes out: if the paint has worn through to the undercoat, or fouling came early, you need more coats or a different paint.
Can I bring antifouling from home and use it in Denmark?
Only if the product is also approved for use in Denmark. Approval is national, so check the product against Miljøstyrelsen's overview of antifoulings that private users may apply before you paint here.
Can I put self-polishing antifouling over hard?
Often yes, if the hard paint is sound and sanded. Hard over self-polishing normally requires removing the old paint or sealing it with primer. Always check the new paint's data sheet.
Can I use ordinary antifouling on an aluminium drive?
No. Copper antifouling makes aluminium corrode. Use a copper-free product made for aluminium drives, with the primer the manufacturer specifies.
Any questions?
Not sure which antifouling suits your boat, your waters and the paint already on the hull? Write to us via the contact page with the boat type, hull material, sailing area and current paint if you know it, and we will help you choose.
