Types of antifouling paint

Mike McKiernan
Posted October 11 2015

There are two 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 in antifouling paint?

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.

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Posted January 12 2018
Re Mike McKiernan's article on antifouling paints; as a marine coatings professional of some 44 years experience, I must correct Mike's statement that there are only 2 types of antifouling. The 2 types described by Mike, "hard" (contact leaching) and "soft" (soluble matrix) are, in recent times eclipsed (both by market share and performance) by self polishing or copolymer antifouling types. Self polishing antifouling can provide multi season growth free performance when applied in a multi-coat system (ideally colour coded) even on pleasure craft with low operating factors. As with traditional antifouling, surface preparation, correct application, recoat windows and most importantly correct cure of the applied antifouling are critical to achieve optimum performance. Again cuprous oxide is the predominant toxin, so many (not all) of these must not be applied to aluminium hulls or sterndrives, consult your chosen antifouling manufacturer.

The most common mistake made is to relaunch your vessel too soon, if a self polishing antifouling is not fully cured when your vessel is re-introduced to the water, it's longevity and performance will be compromised.

Mikes statement that Silicone coating technology (amongst others) "has failed to resolve the problem of fouling" is incorrect, while technically not an antifouling but referred to as growth release coatings these silicones and silicone hybrids coatings are highly effective and proven on moderate to high speed ships and commercial vessels. the low surface tension of these coatings combined with high operating factors and hydrostatic pressure effectively removes marine growth. Silicone hybrids are not viable for pleasure craft other than propellers (provided regular use of the vessel)

To conclude, within reason the better mouse trap is already here, follow these simple rules to give your antifouling a chance of performing to expectations;

1. Prepare existing antifouling correctly.

2. Check antifouling compatibility.

3. Apply at the recommended rate, usually about 6 m2 per litre per coat.

4. For optimum performance (and finish) apply by airless spray (professional)

5. Apply extra film build (coats) in areas of high hydraulic erosion (stem, waterline, prop aperture, rudder)

6. Observe correct recoat and minimum relaunch times, an extra day or 2 on the hard is a cheap investment.
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