TYPES OF ANTIFOULING PAINT
Saltwater is teeming with thousands upon thousands of small creatures swimming along or carried by currents upon spawning. Nature has provided some of these creatures with the ability to feed while they are stationary, like mussels, oysters and barnacles, for example. They feed by ingesting passing particles carried by currents and wave action. To resist the turbulence, these animals have developed a highly sophisticated cementaceous adhesive which they use to anchor themselves to any surface. It’s just like the vacuum created by a rubber suction cup, but with a powerful glue. Once attached, these animals grow and propagate, creating colonies or clusters which in turn captures more passing food… and so on.
For a boat that is constantly in the water, this creates a significant problem. When left untreated, the rudder, propellers, prop shaft and other moving parts will quickly become so encrusted with growth that they will seize up and become inoperable. This growth also creates significant drag to a vessels movement, adding to fuel costs and restricting hull speed.
HOW DOES ANTIFOULING PAINT WORK?
Antifouling is a paint coating that repels these organisms by creating a constantly toxic boundary layer of water next to the hull surface. These paints are designed to prevent the “fouling” of the surface by marine growth, hence the name anti-fouling paint.
Although simple in principle, it has proved extremely complex to perfect, because of the variations of conditions and toxin release rates. In cold climates, toxin release rates were sluggish but so was the growth rate. Conversely, in warmer waters the release rate increased, but so did the propagation of growth, meaning that the anti-fouling paint exhausted its potency much sooner than planned. Once depleted the growth explodes into being. The answer was to produce an optimum “middle of the road” product that would cover both cold and warm water conditions.
Adding to the complexity of these circumstances, the environment also creates a pot-pourri of other intangibles. Rainfall, water temperature, salinity, tidal flows, localised pollution, effluent outflows, and land mass phosphates all contribute to this aquatic “soup”. Then we have photosynthesis which creates the slime, algae and grasses not repelled by the normal toxins. Although confined to the sunlight zone, this development also causes havoc with hull speed and visual aesthetics.
WHAT ARE THE TYPES OF ANTIFOULING PAINT?
There are 2 basic types of anti-fouling paints – hard and soft. Those that allow the release of the toxins through the paint film, and those that shed part of the paint binder/toxins in layers through friction. The latter are referred to as “soft” or “erodible” coatings. The “hard” or “contact leaching” types leave a spent coating of resin binder behind (like holey Swiss cheese) at the time of repainting. This means that the re-coating process builds on top of the existing film. Although this may result in some stripping of the paint layers in 12-15 years time, it also helps to enhance annual repainting because there is normally a “reservoir” of remaining toxin in the old coat to enhance the performance of the new one.
Soft, or erodible coatings, were designed to give commercial vessels an economy of scale.
If a boat had to undergo frequent slippings for routine survey inspections, then it was prudent to use an anti-fouling paint that did not last as long, but was easily washed off and replaced with a new coat, eliminating build-up. Because this type works on friction, it was necessary to impose a speed limit of less than 10 knots. Exceeding that speed simply tore the paint film off too quickly leaving nothing remaining when the vessel stopped. Although technology has now produced high-speed versions of these coatings, they still rely on the shedding process.
Erodible anti-foulings were developed for the international shipping market where continuous hull friction created a “smoother” underwater profile by wearing away the high spots and as a consequence creating substantial fuel savings. This type of product has no advantage for a recreational vessel sitting on a mooring six days out of seven.
HOW DO YOU CHOOSE THE RIGHT ANTIFOULING PAINT?
Here lies a minefield of controversy. Put ten boating people together and raise the topic of anti-fouling and there will be 10 “expert” opinions on which product is best (and which ones are “rubbish”). Then there will be application advice on brush versus roller usage and what “brews” can be made to improve performance (chilli powder, sump oil, antibiotics, anti-fungal additives and so on). These do not enhance the performance of these paints and will affect the controlled release rate of the toxins.
The essential thing with all anti-fouling applications is to not exceed the prescribed coverage rate specified by the manufacturer. The correct film thickness (evenly applied) means that the toxic boundary layer of water will be uniform, and the performance likewise. Cutting corners by inadequate preparation, thinning the paint, or ignoring relaunch time minimums (drying times) will only lead to poor performance and increased costs later on.
Painting propellers require special attention because the vortex created, at speed tears the paint from the leading edges and then continues to work back to towards the shaft. When stationary, the areas without antifouling will attract growth and eventually the whole propeller will become fouled and restrict movement. Only the “hard” types of anti-fouling (like Topflight) should be used. Several coats with emphasis on getting more paint on the outer edge is desirable. Allowing additional drying time is also worthwhile. Painting the propellers first will enable more coats to be applied and provide the longest drying times.
WHAT ARE THE TOXINS?
All anti-fouling paints contain copper as the main repellent. There are two forms of copper: Copper (cuprous oxide) and copper (cuprous thyocianate). The latter is less corrosive to aluminium than the full blown cuprous oxide, but corrosive nonetheless. Other additives to control slime/weed only play a minor role. The main problem is caused by barnacles, tubeworm and teredo worm – only copper is a satisfactory deterrent.
This copper is released into the water by a controlled leaching process called hydrolysing. This rate is 2.5 to 7 micrograms (millionths of a gram) per square centimetre per 24 hours. Within this range the paint will remain active for as long as the copper is being released. When exhausted, the marine growth will start to re-established within hours. The higher the copper content of the paint the longer it will take to be released, which translates into longer periods between slipping.
SELECTING THE BEST ANTIFOULING FOR YOUR BOAT
Any vessel doing 10 knots or over should not use Soft Copper anti-fouling’s.
Boats made of aluminium, or those with stern drives or sail drives must not use Norglass Antifoulings. Select a brand containing copper Thiocyanate and follow the manufacturer's instructions to the letter.
Boats doing less than 10 knots should seriously consider upgrading to the hard (contact leaching) anti-fouling’s as value for money is obvious. (Topflight Red, Black or Blue).
Craft that use a travel-lift or crane for haul-out should not use Soft Copper anti-fouling’s because the slings will tear the paint off upon re-entry. The paint is too soft.
Soft copper anti-fouling’s can only have more Soft Copper paint applied over them (brand irrelevant). To use a hard type, the old soft copper must be totally removed.
Hard type anti-fouling’s are generally compatible brand to brand and, given normal preparation, not a problem.
Active constituents of all anti-fouling’s must, by law be stated on the main panel of the label. Comparing the price and the stated grams per litre (g/l) can influence the choice of product.
Considering that there are only a small number of manufacturers of anti-fouling paints worldwide, and with a vast tonnage of product used every year, (more than a million litres of anti-fouling on commercial shipping vessels sitting off Singapore harbour any day of the week!) illustrates the magnitude of the problem.
Globally, scientists have tried Silicones, Teflon, Lanolin, synthetic compounds and an assortment of other ideas to resolve the problem of fouling without success. With a lead time of seven years (minimum) to produce a new anti-fouling paint, plus the investment in R & D, making a better mousetrap is a hard ask.
HOW TO REMOVE OLD ANTIFOULING
On surfaces such as steel, aluminium or wood, the most practical ways are either by paint stripper, soda blasting, wet sandblasting, or a “poultice” technique which uses a paper over a coated solution and then gets peeled off. Last option is to wet sandpaper the surface. Because the toxins are potentially hazardous, dry sanding is not an option. Any removal of old anti-fouling paint must be done responsibly, so that all of the removed material is contained, or captured before it has the opportunity to reach the water.
Removal of anti-fouling paint from fibreglass can cause some additional headaches because the use of chemical paint strippers can affect the integrity of the gel coat. If a slow reacting paint stripper is used, only small areas should be coated at any time so that the paint/residue can be removed before the stripper has time to react with the gel coat surface.
HOW TO STORE ANTIFOULING PAINT CORRECTLY
Anti-fouling paints are normally very dense (heavy). A can of Topflight Red for example weighs-in at more than 2kg per litre. As a result the copper settles in the can faster than other paint products. In order to keep the copper in suspension, the tins should be turned upside down once a month to prevent compacting of the contents. Meticulous stirring is also essential, right up to the time of applying. If all of the toxin is not totally integrated within the volume a patchy result can occur.
Consider this: If a tin of anti-fouling has been sitting on a shelf for a few weeks, the weight of the copper will want to sink lower in the can. Therefore, if the tin is opened and just painted on, the first 20% of this product will have a lower copper ratio. Then as the residue arrives at the surface, it will be disproportionate with a copper sludge, and less formulated material, to provide the correct rate of dissolving. If this happens there is no way of providing an overall controlled release rate of toxins. Some areas may begin growth within days/weeks.
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