Toxicology Types of anti-fouls Compatibility Application Stern drives, props and anodes Health and safety Shopping around The future
- While copper biocides are not as effective as tin, they are the best alternative and will remain so for the foreseeable future
- The latest advance in anti-foul coatings has seen the introduction of copolymer chemistry
- The advantage of copolymer technology is that so long as you have paint on the bottom you have antifouling protection
- Ensure that any new anti-foul coating being applied to the hull is compatible with the existing anti-foul
- Professional application generally includes better preparation, a more uniform film thickness, a smoother finish and better masking of the waterline and boot top
Anti-fouls are a mixture of a binder (paint that carries the colour) and a biocide. The biocides kill marine growth while the binder essentially holds the formula together and provides a quality cosmetic finish in a variety of colours.
Binders range from the old oil-based paints to modern vinyl paints. But, for practical purposes, there are only two biocides – those based on copper and those based on tin.
Toxicology
Back in the 1970s industrial chemists pretty much had free reign in determining what biocides they used in their antifouling formulas.
Tributyltin (TBT) – essentially tin – was the favoured biocide, although it was most commonly used in conjunction with other biocides like copper. It became evident in the 1980s, however, that TBT was causing severe damage to shellfish communities.
This led to a 1987 Europe-wide ban on TBT in anti-fouling paints on boats under 25 metres. It is still permitted in some countries on vessels over 25m and in some industrial and agricultural applications. TBT is still regarded as the most effective biocide and chemists have struggled to match its performance.
The TBT ban saw a shift back to copper as the main biocide. This is commonly used in the form of cuprous oxide but also as cuprous thiocyanate and metallic copper powder.
The main disadvantage with copper-based anti-foulings is that the copolymers are not as effective when used on their own. They are also incompatible with aluminium hulls and restrict the colours that can be produced.
While copper biocides are not as effective as tin, they are the best alternative and will remain so for the foreseeable future. To achieve the best performance possible a secondary booster biocide is routinely used.
The latest advance in anti-foul coatings has seen the introduction of copolymer chemistry. Unlike conventional paints, the biocide and the binder are combined into a single molecule. This has seen the development of self-polishing anti-fouls like International Paints Micron 66. International’s Australasian technical manager Keith Ryman says the copolymer reacts with salt water, sustaining the release of biocides throughout the lifetime of the coating.
Because the surface of antifouling is constantly renewed by chemical reaction, it continually smoothes to provide a low-friction surface. The technology works particularly well for vessels that remain stationary for long periods of time. But, like other anti-foul brands, Micron 66 has a finite life of 12 to 18 months.
Conventional anti-foulings work by allowing minute quantities of biocide to leach out of the binder. The leaching process supplies enough toxins to kill most marine growth until no more biocide can reach the surface of the paint. The problem is that leaching effectively stops before the paint shows signs of deterioration. The advantage of copolymer technology is that so long as you have paint on the bottom you have antifouling protection.
Types of anti-fouls
Hard anti-foul
Hard anti-fouling, when immersed, is hard enough to withstand regular wiping without removing substantial quantities of paint. Hard anti-fouls have long been favoured by the racing fraternity because the anti-foul can be scrubbed prior to a race for maximum performance. It is far less effective on vessels that are not used from one month to the next.
Hard anti-fouls tend not to wear much and become unsound and fail to retain sufficient strength to allow new coatings to be reapplied directly over the top. This means the anti-foul has to be completely stripped from the hull after several seasons.
Eroding anti-foul
These wash and wear away until there is no anti-fouling left on the hull. This can take some years to occur but don’t be fooled. The current generation of eroding anti-fouls stop leaching biocides long before the paint shows visible signs of deterioration.
This occurs because as the paint film starts to get thin, the biocides are progressively washed out. This typically occurs after 12 to 18 months. While eroding anti-fouls are particularly suitable for low-use vessels, they are not recommended for high-speed powerboats.
Compatibility
Ensure that any new anti-foul coating being applied to the hull is compatible with the existing anti-foul. The simple rule is that eroding anti-fouls may be applied over both hard and eroding anti-fouls. Hard anti-fouls must not be applied over eroding anti-fouls.
Your hull should be in good condition, with little or no damage. You will need to give the surface a wet sand with 80-grit sandpaper. This removes the top layer of paint that is depleted in biocides, contains salt and calcium deposits. Failure to remove this layer will lead to blistering, delamination and poor anti-fouling performance.
If the paint is an unknown product, after sanding and washing allow it to dry and apply a sealer or tie coat to avoid any unwanted interaction between old and new and to provide a surface that the new anti-fouling can bond to.
But don’t apply a tie coat over old unsound anti-fouling. Before long the new anti-fouling will simply fall off.
If the hull has damaged or bare areas these will require re-priming.
Anti-fouling application
Before purchasing anti-foul, work out the area of your hull. Insufficient film build-up is the single largest cause of premature failure. Most manufacturers recommend two coats of anti-foul if applied by brush or roller, and it is a good idea to give foils and (rudder, skeg and keel) and high turbulence areas (waterline) an additional coating.
While most anti-fouls can be thinned up to 10 percent it is not recommended. It is also particularly important to stir anti-foul paints vigorously. Heavy metals typically settle to the bottom of paint tins and must be mixed thoroughly with the binder.
Short mohair rollers generally ensure an even thickness of paint and large areas can be covered reasonably quickly. Remember, don’t put too much paint in the paint tray because the metals will settle to the bottom, and in hot conditions regularly stir the paint.
Rollers can be difficult to use in some areas, and a wide 5” brush will be required. Brushing high-flow areas and foils is also recommended to ensure adequate film thickness.
However, brushes tend to lose hairs and bind, requiring regular cleaning with thinners. The roller/brush combination is best. Masking waterlines demands careful attention.
Avoid regular masking tape as the chemicals in anti-fouling have a habit of leaching into the tape glue, giving a furry finish. Fine line tapes are best. These should be removed soon after the last coat of anti-foul has been applied. Remember to leave a little anti-foul aside to brush onto the bottom of the keel and where the cradle arms have prevented painting on the hull.
If the vessel is to remain on the hard after anti-fouling try and protect the new anti-foul from direct sunlight. Sun can cause some ingredients in the paint to oxidize. It also pays to lightly wet sand the surface with 400-grade paper prior to relaunching.
Stern drives, props and anodes
Diesel and petrol engine sterndrives are built from aluminium and these must not be coated in copper-based anti-foulings. The same applies to aluminium-hulled vessels and aluminium propellers. Cuprous Thiocyanate/cobiocide anti-fouls must be used instead.
Bronze propellers can be painted in copper-based anti-fouls but with mixed results. These days there are a number of single and two-pot products specifically designed for propellers, shafts and A-brackets. Anodes should not be coated.
Health and safety
When handling any of the above-mentioned paints remember to wear the appropriate safety gear. Even when wet sanding old anti-fouling care should be taken to avoid splashes in the eye or on bare skin. And again, never dry-sand anti-fouling of any type.
Shopping around
Typically two thirds of the total bill will go to the marina, getting the boat in and out of the water. But, shopping around is well worth the effort. Environmental restrictions have all but done for tide grids.
Many marinas offer winter anti-fouling specials. These generally include a return lift and haul, water blast, wet sanding of the old anti-foul, masking and airless spraying of the new anti-fouling and three days on the hard stand. Professional application generally includes better preparation, a more uniform film thickness, a smoother finish and better masking of the waterline and boot top.
The future
Environmental attention is now focused on the continued use of copper in anti-foulings and there is an expectation that copper will be banned within the next two decades. Chief among investment and research are anti-foulings of organic origin. These tend to be less effective than other biocides, may deter only specific types of fouling organism, and are expensive to develop.
The other approach is totally non-toxic systems. These include non-stick type coatings better known as fouling release or minimally adhesive surface coatings. They work by reducing the ability of organisms to stick to the hull surface, so that when a vessel moves through the water the fouling growth is washed away. It is not considered, however, to be particularly effective when used without a biocide.
There has also been success with biocide-free silicone rubber coatings and these are now routinely applied to fast passenger ferries and tourism vessels. The cost of the coating and its application is such that silicone coatings are unlikely to appeal to the recreational market.
To read in-depth boat maintenance stories, see the latest issue of Trade-A-Boat magazine, on sale now.