How to Remove Barnacles From a Fiberglass Hull
The fewer the barnacles, the less likelihood there is of damaging the hull while removing them. In other words, if you wait until the barnacle infestation has encrusted much or all of the hull, you'll be faced with hours of heavy duty removal work, and probably need to completely sand and repaint the hull once you're done.
Hulls with Light Barnacle Buildup
If your hull has only a few barnacles or patches of very small barnacles scattered about, then it's time to get scraping.
Pressure washers aren't very effective on young underdeveloped barnacles, and you're more likely to damage your hull's paintwork or gelcoat by using one. When scraping to remove barnacles, use plastic scrapers or wooden spatulas, which cause less damage to the hull than metal scrapers.
1. Work the scraper under the edge of the barnacle to prize it off. Keep the scraper as flat to the surface of the hull as possible to avoid gouging into the fiberglass.
2. Once removed, barnacles leave behind a circular calcium foundation known as a husk. Carefully remove as much of the husks as possible with the scraper without damaging the gelcoat or paint.
3. Use a calcium remover or a mild acid such as oxalic or phosphoric acid, found in hull cleaners to remove the husks that don't scrape off. Apply the chemical to the husks, let it penetrate them, and rinse away with water. A couple of applications and gentle scrubbing with a nylon brush may be necessary to remove the husks.
4. Apply a compound and polish the gelcoat if the hull was damaged while removing barnacles to restore the finish.
5. When all the barnacles and husks are removed, clean the hull with a fiberglass-safe boat wash, wait for it to dry, and then apply at least two coats of boat wax, which slows the reattachment of marine growth and makes removing new barnacles much easier.
Hulls with Heavy Barnacle Buildup
If your hull has been in the water for a long time and there is considerable barnacle growth covering large areas or the entire hull, you'll need to hit it with everything you can, and you'll likely have to repaint the hull once you're done.
1. Use a pressure washer to remove as many of the barnacles as possible. You'll need a pressure washer capable of at least 3000 PSI and a small nozzle for maximum power. Be particularly careful not to gouge or cut into the fiberglass using these types of nozzles, and spray parallel to the hull's surface with the pressure washer.
2. Scrape away the remaining barnacles and as much of the husks as you can. Start with a plastic scraper to minimize damaging the fiberglass, but if the barnacle buildup is severe you may have to resort to a stronger, stiffer metal scraper.
3. Apply a calcium remover or a mildly acidic hull cleaner to dissolve the remaining parts of the husks. Use a nylon brush to work the chemicals into the husks, and be prepared to repeat this step several times to remove any last traces. In extreme circumstances, hull cleaner or calcium remover may not be enough to remove thick beds of husks, in which case muriatic acid or HCI may have to be used. These are much more caustic acids, and the utmost care must be taken to protect your boat from permanent damage. It's recommended that muriatic acid/HCI only be used by professionals.
4. Once all the barnacles and husks are removed, wash the hull and assess the damage. If the hull is relatively free of any deep gouges or scrapes, you may be able to compound, polish and wax the hull back to its former shine. However, if the damage from scraping and power washing is deeper, it'll need to be filled with resin, and the hull will need to be sanded and repainted.
5. Apply at least two coats of antifouling bottom paint to the freshly cleaned hull. The more coats you apply, the longer the paint and its effects of repelling marine growth will last. A quality anti-fouling bottom paint will also last longer and be more effective, so spending a bit extra on a better paint could save you a lot of future work.