Bottom paint and antifouling: rules, biocides and environmental considerations

Everything that lies in water grows to. Algae, seaweed, and other organisms settle on the hull, increase water resistance and reduce speed, and that is why almost all boats below the waterline get an anti-fouling paint. But anti-fouling paint is not just paint – it is a biocide-containing product, and the rules for what you can use and where are quite precise.

§How primer paint works

Anti-fouling paint contains a biocide, an active substance slowly released from the paint that makes the surface unattractive or toxic to fouling organisms. According to the Environmental Protection Agency, copper compounds, such as copper thiocyanate and copper(I) oxide, are among the most commonly used active substances in anti-fouling paint in Denmark, often in combination with zinc pyrithion. Some paint types are self-polishing and wear slowly away with an even layer of biocide as a result, while other types stick more firmly and are renewed by cleaning.

§Biocidal Products Regulation and approval

Bottom paint is regulated by the EU's Biocide Regulation. An active substance must first be approved at EU level and then a company must apply to the Environmental Protection Agency for approval of the specific product before it can be marketed in Denmark. The approval includes an assessment of both environmental and health hazards and whether the product actually works. This means that as a professional you should only use bottom paint that is on the Environmental Protection Agency's list of approved products — not imported or home-mixed products without approval.

§From TBT ban to modern rules

Bottom paint has a heavy history. For many years, tributyltin (TBT) was used as an extremely effective biocide but the substance was found to seriously damage the marine environment including by causing sex changes in sea snails even at very low concentrations. TBT was therefore banned internationally through the IMO Convention on Harmful Antifouling Systems and today's rules are a direct consequence of that experience: active substances must be documented and approved before they reach the market.

§The boat mechanic's tasks

• 01Clean old bottom paint responsibly — sanding dust can contain heavy metals and must be collected, not swept into the harbour basin
• 02Use appropriate respiratory protection and gloves during sanding and application
• 03Check dimensions against drawing before cutting
• 04Follow the manufacturer's guidelines on coat thickness, number of layers, and drying time
• 05Dispose of residues and grinding dust as hazardous waste

§Practical application

The bottom is sanded or washed clean of loose, old paint, any damage to the gelcoat or primer is repaired and the paint is applied in the number of coats the manufacturer prescribes — typically several coats at the waterline and in high-wear areas such as the leading edge of the keel and rudder. Temperature and humidity affect curing and many manufacturers specify a window for when the boat may be launched earliest after application.