Should you thin antifouling paint?

Proper application is key to getting the best result from any antifouling paint. There is some debate as to whether you should thin antifoul paint. Most manufacturer will tell you not do it, claiming that their antifoul are formulated to the optimum viscosity for their designed application thickness.

In my opinion, they discourage you from doing using thinner for the following reasons:

  • they are concerned you will not use the correct type of thinners for the paint. Not all thinners are born equal ! Some are fast evaporating some are slow to eveaporate and some never evaporate !! You could use anything from a long list of thinning chemicals: white spirit, acetone, turpentine, naphtha, toluene, methyl ethyl ketone (MEK), dimethylformamide (DMF), 2-Butoxyethanol, ethylbenzene, xylene, n-butyl acetate, butan-1-ol… The companies’ s chemists are not keen that you trash their careful formulation by adding willy-nilly some stuff that says thinner on the bottle, that you picked up at your favourite diy store because it was on special offer….

    Most antifoul manufacturer whilst telling you not to thin the paint – in fact sell a special thinner for their paint. For example International sells Thinner No.3 for antifouling paint. If you use the manufacturer’s thinner – it will be the correct thinner for the paint. If you balk at paying 18 Euros for 500 ml of this wonderful product …. You need to try to get your hands on some heavy naphta (no, not light naphta and no it is not the same thing as white spirit..). You are unlikely to find it at your local diy store… In fact the stuff is practically impossible to get in Europe (like a lot of chemicals these days)… If it is cold, when you are painting the bottom of your boat (between 5 et 15 °C ), you could get away with using Xylene which you might find at your local hardware store, perhaps labelled as metal cleaner… Basically just use the manufacturer’s thinner

  • most people greatly over thin paint. They dilute it upwards of 20% and 30% which dramatically reduces the film’s effectiveness as an antifoul and it’s ability to stick to the surface.

  • most people want to thin antifoul for the wrong reason. You should not thin antifoul paint to be able to apply it more quickly and easily. You should not thin antifoul paint to make it stretch – increasing paint coverage to get away with not buying that extra tin of paint. As well as having bad film formation – you’ll just end up with less paint and less biocides and your antifoul protection might not last an entire season…..

Why and how to correctly thin antifoul paint

The reason to thin antifoul paint is to achieve a smoother bottom finish.

Antifouling is normally quite viscous (this is because of the richness in pigments and biocides, a high viscosity is needed to avoid settling and syneresis); and behaves as a pseudoplastic – meaning that when you stir it the viscosity drops (it becomes more liquid) but bounces back as soon as you stop (the paint thickens again). This behaviour means that if you apply it with a brush, the brush strokes have a tendency to congeal immediately and to be visible. On the positive side the paint is less runny and drippy… This pseusoplastic behaviour is what makes achieving a very smooth finish with either a brush or a roller very difficult. This is why we want to apply thinner. We want to shift the pseudoplastic threshold to obtain a film of paint that will shrink slightly when drying, giving a smoother finish. To do this we need a heavy thinner which will slow down the drying time by 20 – 40 minutes…

You should be able to achieve this by following the following instructions:

  1. Use the manufacturer’s recommended thinner
  2. Dillute your first coat 5 - 8%
  3. Dillute your second coat 10 - 15 % to achieve a very smooth finish
  4. Apply a third coat around the waterline