How do you apply antifouling paint to a boat?
I need to antifoul my 34-foot sailing cruiser this weekend. The old paint is eroding but mostly intact, though there's a bit of slime and a few barnacles near the waterline. What's the best way to prep the hull and roll on the new coat without making a massive mess?
Answer
Apply antifouling by thoroughly wet-sanding the existing coat, masking the waterline, and rolling on two fresh coats of marine bottom paint using a solvent-resistant roller. The finish relies entirely on preparation, meaning you should spend most of your weekend washing, scraping, and sanding before opening a single tin.
Preparing the hull
Start by power-washing the hull as soon as the boat is lifted before the slime and weed dry out. Place a heavy-duty plastic tarpaulin under the keel to catch paint flakes and sanding slurry. Most British boatyards now strictly enforce environmental rules regarding toxic runoff. Once dry, tackle hard growth like barnacles or tubeworms using a tungsten carbide scraper. A 50mm Bahco scraper works well here.
After scraping, abrade the entire hull to give the new paint a mechanical key. Never dry-sand antifouling paint because the dust is highly toxic. Always wet-sand using a medium abrasive pad or 80-grit wet-and-dry paper. Wear a P3 half-mask respirator alongside a Tyvek suit. Wash the hull down thoroughly with fresh water to remove the toxic slurry and let it dry completely. If flaking areas expose bare gelcoat or old epoxy primer, feather the edges and spot-prime these patches with a compatible underwater primer like International Primocon.
Masking and mixing
To get a crisp waterline, run a continuous length of high-quality masking tape along the boot top. Standard masking tape turns to mush in the damp yard air and leaves stubborn adhesive behind. Use a UV-resistant marine tape like 3M 2090 blue tape. It costs around £8 a roll but saves hours of cleanup. Keep the tape about 2mm above the old antifoul line to ensure you overlap the joint.
Next, prepare your antifouling. High-quality ablative paints like International Micron 350 or Seajet 033 Shogun cost £100 to £140 per 2.5-litre tin. They contain heavy copper and biocides that settle into a dense sludge at the bottom of the tin. Do not just give it a quick stir. Use a flat wooden paddle or a drill-mounted paint mixer on a slow speed. Spend at least five minutes scraping the bottom and blending until the consistency is completely uniform.
Rolling on the paint
For application, use a solvent-resistant medium-pile roller. A 9-inch cage is fast for the wide flat sections of a motorboat hull. A 4-inch radiator-style roller is often much easier for the curved bilges and fin keels of a sailing yacht. Buy a multipack of sleeves. They will degrade after a few hours in the heavy solvent.