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Anti-Fouling Bottom Paint
Anti-fouling bottom paint can help keep your boat's hull in good condition, prevent barnacle attachment and algae growth, and will help your boat move through the water faster and more efficiently.
Bottom fouling results from 3 primary causes:
Does Your Boat Need Anti-Fouling Bottom Paint?
Not every boat necessarily needs anti-fouling bottom paint, and the type of bottom paint that is best depends on water conditions and the intended use of the boat. In some places, hull fouling occurs so quickly that boats can only be left in the water a couple of days before noticeable (and difficult to remove) growth can occur, while in other places boats can be left longer without bottom paint. While it is safe almost anywhere to leave a boat in the water for a night or two without bottom painting it, it is best to ask other boaters in your area about the common boat bottom fouling threats and how quickly they occur.
Types of Anti-Fouling Bottom Paint
Anti-fouling bottom paints use biocides, usually cuprous oxide or copper thiocyanate, to discourage marine organisms from growing on the hull. TBT (tributyltin) was a commonly used biocide, but is illegal for most common purposes now. Some anti-fouling paints also have anti-slime and soft growth inhibitors like Biolux and Irgarol to further discourage algae and underwater plant life. Ablative bottom paints are designed to be applied in a fairly thick layer that wears away like soap, constantly exposing fresh paint. Others are hard epoxy or vinyl paints impregnated with biocide, which leaches out and leaves the depleted paint on the hull. There are also a few made with TBT substitute tin-based biocides and others, and bottom paints made with no biocide at all that are so slick that organisms cannot attach themselves firmly and can be wiped off.
Boaters faced with the necessity of a bottom job have to answer a number of questions. Should I buy ablative or hard epoxy bottom paint, or a harder advanced soluble copolymer type ablative? How long will the paint last? How will it affect speed and efficiency, both initially and over the lifespan of the paint? Which bottom paints can be used on top of which others, or on inflatable boats or aluminum hulls? Are sailboat racers who apply an extra $2,000 worth of Baltoplate just to sand it smooth and burnish it with a newspaper insane? Only the last one has a definitive answer: yes.
Ablative & Soluble Copolymer Bottom Paints
The older ablative paints were made to wear away through friction as the boat moved through the water, and some of these paints are still available today, such as Interlux Bottomkote ACT and XXX. The problem with these paints is that water wears the paint away much more quickly in areas of high friction, such as the leading edge of the keel and waterline, leaving those areas exposed while there is still plenty of good paint on the rest of the hull.
Modern ablative paints with water soluble or self-polishing copolymers dissolve at a predictable rate, and are not as susceptible to friction wear so tend to wear more evenly and last longer. Paints such as Pettit Ultima and Interlux Micron 66, Micron Extra and Micron CSC, as well as Awlgrip Awlstar, Seahawk Cukote, Super Shipbottom, paints by SeaJet and others fall into this category of more advanced ablative type paints. Pettit's Vivid is a special case, and has characteristics of both ablative and hard paints. The copper thiocyanate biocide and zinc slime-inducers do dissolve away like an ablative bottom paint, but they leave behind the Vivid paint, as happens with a vinyl or epoxy antifouling coating.
Hard Bottom Paints
Hard bottom paints like Pettit Trinidad SR and Unepoxy, Interlux Ultra and Fiberglass Bottomkote, Seahawk Sharkskin and Tropikote and others are durable and can be made very smooth. It is possible to achieve and maintain a smoother finish with these bottom paints than with most ablative types and the paint is long-lasting.
The downsides of the hard paints are that many lose effectiveness when out of the water, and the paint will build up over time if not sanded down each time the boat is bottom painted. Also, hard paints are at their most effective immediately after launching, and are less toxic to marine growth over time as the poisons within leach out. That means that after a year or so, a hard bottom is likely to need more scrubbing than an ablative bottom, which will still just need a light wipe to expose fresh paint with copper biocide.
High Performance Bottom Paints
Racing sailboats and powerboats frequently use special bottom paints with Teflon to minimize hull friction. Common choices for fresh water use are Interlux VC 17m and Pettit SR-21, while salt water racers use Interlux VC Offshore or Baltoplate. These paints are generally incompatible with other paints, so if some crazy racer has put VC 17 on a hull you bought, you will have to remove it before applying a different type of bottom paint.
The racing paints must be applied correctly in thin layers, then wet sanded with increasingly fine sandpaper, then burnished with cloth or newspaper until it shines to produce their maximum performance finish. Vivid is also used by racers, but the zinc in it produces a smooth coating of slime. It just needs any growth wiped off. High tech racing teams also commonly use Durepoxy, a biocidal epoxy bottom paint for high performance boats.
Special Purpose Bottom Paints
There are also a few special purpose or unique antifouling paints, such as EP 2000 by ePaints. This copper free paint uses different biocides and generates hydrogen peroxide around the hull to discourage marine growth. It is safe for aluminum, as is Interlux's Trilux 33 and Pettit's Alumacoat SR. There are also biocide free super-slick paints that are simply too slippery for marine growth, and manufacturers are working on spray-on glass surfaces. Inflatable boat antifouling paint is specially formulated to stick to hypalon rubber and flex with it without peeling off.
Choosing the Right Bottom Paint
There is a bottom paint for every purpose and the right one for your use depends on laws in your area, the previously applied bottom paint or hull surface material of your boat, how much work you are willing to do, how fast you want to go, how much you want to spend, and how long you want it to last, among other factors. It's a very personal choice, a little like choosing the boat itself, but slightly less permanent and expensive.
Most of those questions can only be answered by the boat owner, but the bottom paint compatibility question can be easily solved by examining the manufacturers' compatibility charts. These charts show which paints can be used over various other paints, or on bare fiberglass, wood, or metal hulls, and the type of surface preparation that is recommended. Some require only a light sanding and solvent wipe down, while others must be completely stripped before the new paint can be applied.
Estimating Bottom Paint Coverage Requirements
Once you determine the paint that is appropriate for your use, you will need to estimate the amount required. Bottom paint is expensive, and extra paint is toxic and often presents a disposal problem, so you do not want too much, but of course you don't want to waste time and money by running out before the job is done. A safe rule of thumb is to multiply the length of your boat by the beam to get the square footage and buy enough to cover that area the recommended number of times, using manufacturer's estimates of bottom paint square foot coverage.
If you have extra bottom paint, you can apply it to areas like the leading edges of the keel or rudder that are prone to wear, or put an extra coat around the waterline, where growth tends to be heavy. With bottom paint and any needed primers, solvents, etc. on hand, it is time to proceed to Preparing Your Boat for Bottom Paint.
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