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Antifouling is best applied on a dry, calm day. It is best to apply the antifouling in the middle of the day to ensure the hull is dry and as warm as possible.

If you have removed all the old antifouling layers back to the gelcoat, then it will be necessary to apply a coat of primer before painting on the antifouling. The reason for doing this is that the antifouling will not adhere properly to the gelcoat.

If a GRP hull is stripped back to the gelcoat then you should consider applying an epoxy coating to seal the hull and protect it against osmosis. It is worth getting professional advice about this as if your boat’s hull has high moisture readings it is best not to seal in the moisture with a layer of epoxy. You should dry a hull out as much as possible before applying the epoxy. It is tricky stuff to use and to be fully effective the hull needs to be very well prepared. Two or three epoxy layers are applied, and overcoating times vary according to the conditions. Each layer has to be allowed to dry to the point that it is touch dry rather than completely hard and then the next layer can be applied. Like most other jobs like this, preparation is key, as is following the manufacturer’s instructions.

hull keel antifouling, antifoul paint

Applying antifouling

Minimum application temperatures vary so check what your paint manufacturer recommends. A common minimum is 5ºC, but note this would need to be the temperature of the hull. If the paint is applied in too low temperatures, it may not flow over or adhere to the existing paint. Likewise, painting on a windy day is not advised as the paint will more likely become sticky and dry too quickly.

Make sure you have everything you need including:

  • Antifouling for two coats.
  • Paint stirring stick.
  • Protective clothing – goggles, gloves, respirator /breathing mask for solvents (not a dusk mask), overalls.
  • Plastic sheeting to protect ground from spillages.
  • Masking tape.
  • Thinners as recommended by the paint manufacturers.
  • Stick to stir the paint.
  • Long and short handled rollers + pack of gloss roller pads (not emulsion ones).
  • Roller trays.
  • Brushes.
  • Paper towels and or rags.
  • Rubbish bag for disposal of used rollers/trays/masking tape/gloves.

 Step 1

Before painting, apply masking tape along the waterline. I find it best to use a good quality plastic masking tape, as there will be no seepage of paint beneath the tape and it will be easy to remove the tape after you have finished painting. Cheaper masking tape can be very hard to peel off if left for more than a few hours and you can end up with an uneven edge. Take care to keep the masking tape straight, working it along the existing waterline without it kinking. Also mask off anything that does not need antifouling, including sacrificial anodes, transducers and the log impeller.

Step 2

Stir the paint well using a wide stick. This will take several minutes as the biocides are thick and heavy and need to be mixed well into the paint. When you are satisfied it is completely mixed, pour some into a paint tray and reseal the lid. The last thing you want to do is step back to admire your handiwork and kick a tin of antifouling over by mistake. It is a good idea to stir the paint each time you re-fill the tray as it can get very thick and sticky near the bottom of the tin otherwise.

Step 3

Be generous with the paint. Go back after you have finished the first coat and add extra layers along the leading and trailing edges of the keel and rudder. Use a roller as much as possible but also have an old brush to reach the inaccessible places. If it dries out quickly, then you can paint a second coat of antifouling on the same day, but I usually come back the next day to do the second coat.

Step 4

If your boat is supported by cradle pads, then to do a proper job you need to paint beneath the pads. There are a couple of options for doing this. One is to ask the boatyard to move each of them just enough to enable you to come back and paint the patches. The other option is wait for the boat to be lifted into the slings before launching and painting the patches on the day of the launch. The second option does not normally involve an additional cost, while the first option usually incurs a yard labour cost, which is fair enough.

 Step 5

Peel away the masking tape, dispose of all the used rollers, brushes, tape and debris, step back and admire your handiwork.

 Antifouling keels

Exposed keels (as opposed to encapsulated keels) require special attention as they are prone to corrosion. Most keels are made from either steel, iron, lead, or a combination of a steel fin and a lead bulb. Note that lead does not rust but it does need antifouling and requires an underwater primer to be applied.

 Keels in good condition

  • Before applying coatings, pressure washing is needed to remove as much loose material as possible.
  • Abrade with 60 or 80 grit wet and dry sand paper used wet to prevent dust.
  • Remove all residue and wipe clean with fresh water.
  • Apply underwater primer as soon as possible, before the surface begins to oxidise. Usually at least three coats are recommended.
  • Finally apply at least two coats of the same antifouling as used for the hull.

Antifouling boat hull

 Keels in poor condition

  • If there are signs of rust, pitting or small cracks along the keel surface, then you will need to remove all of the surrounding paint.
  • Use an angle grinder, wire brush or shot blasting to take the keel back to bare, shiny metal.
  • Next apply a rust preventer.
  • If the metal is seriously pitted, then the holes need to be filled. There are epoxy products available to fill cracks and holes in iron keels.
  • After filling with epoxy, fair the surface using 60 or 80 grit paper.
  • Then apply at least three coats of underwater primer or as recommended by the manufacturer.
  • Finally apply at least two coats of the same antifouling as used for the hull.

Keel maintenance

Keels are designed to act as underwater foils that generate lift as the boat moves through the water, counteracting the leeward force of the wind and enabling the boat to sail closer to the wind. Keel maintenance and repair is essential for the performance of your boat.

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