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Photo by Roger Marshall
Photo by Roger Marshall

Bottom Painting

It is the time of year when most sailors are painting their boats’ bottoms. Many like to do the job themselves but have questions when choosing and applying the right paint.

Begin by answering these four questions.

How fast is your boat?
If you have a fast boat, maybe a 20- to 30-knot powerboat, you’ll need a “hard” bottom paint (known as a contact leaching paint) or the impact of water on the boat’s bottom will simply wear it away, leaving you with no protection.

Ablative paints wear away as the boat goes through the water. These paints are best on mid-range and moderate speed craft, such as sailboats. They slowly erode and will eventually disappear. This says that the paint thickness is the governing factor in how efficient the anti-fouling is.

Self-polishing copolymer (SPC) paints wear away in a manner similar to ablative paints, but the antifouling chemicals are bound in with the paint layer and only react with salt water. Note that only the layer of paint in contact with seawater reacts. Bottom layers are unaffected until the paint erodes enough. For this reason, a boat painted with SPC paint can be hauled and relaunched without having to be repainted.

Boats that move slowly or stay in one place can use the older (and cheaper) rosin-based paints. In most cases, these paints require that the boat be launched within 24 hours of painting.

How foul is the water in which your boat floats?
If the water is very foul, such as in bays where rivers meet the ocean, you need a paint with lots of biocides. Copper is a biocide, but today’s paints contain more than just copper. These days, Irgarol, zinc and other biocides are added to bottom paint to minimize the amount of copper and to reduce the cost of the paint.

When a boat first goes into the water, the paint works properly for a month or so, then a layer of slime builds up. This slime layer slows the leaching of the copper biocide and eventually marine life gains a foothold in the slime layer. As “cling-ons” become more prevalent, the biocide cannot reach the outer layers to stop cling-ons gaining a foothold. By preventing the buildup of slime using products such as Biolux and Irgarol, the copper biocide works much more effectively, over a longer period than a paint without it.

How warm is the water where your boat is?
Warm water breeds more cling-ons, so you’ll need a stronger paint. Warm water in estuaries and tidal flats has high levels of growth so your paint will need a high level of biocides.

How often do you use your boat?
If you use your boat regularly, you can probably use an ablative paint. One of the big advantages of ablative paints is that you don’t have to sand them off at the end of the season – they should have eroded away. If you dry sail your boat or trail it regularly, you should get an antifouling that can be used year after year without recoating. Many antifouling paints oxidize when the boat is hauled out of the water, which means they stop working. A few are formulated to work every time they are put in the water and stop working when they are taken out. Trailer owners should look for a multi-season paint.

This table lists paints from the major manufacturers. A few additional brands from other manufacturers may also be available. It does not include store labeled brands which might be made by one of the majors and relabeled.

 

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