Antifouling paint has made boat bottom maintenance easier, but antifouling paints can fail for a number of reasons. If you understand what causes the problems, it's possible to prevent some of them from happening.
Improper surface preparation
Adhesion failure, peeling, flaking, and delamination problems occur due to improper surface preparation. If gelcoat blisters are present, they must be repaired first. Before painting, be sure the surface is clean, dry and free of contaminants.
If peeling occurs on underwater metal parts, it's most likely the paint was applied to dirty metal or an improper primer was used on the metal. Underwater metal parts such as the prop shaft should be cleaned back to bright metal by sanding and solvent wiping.
Insufficient paint thickness
For best results, heed the paint manufacturer's recommendations for film thickness and recommended number of coats. Antifouling paint is typically much thicker than topside paint, for example. Resist the urge to thin it in order to make it spread further. You won't be saving money and you'll reduce the effectiveness of the paint. If you don't apply the recommended number of coats, you risk affecting the longevity of the paint.
Missing the overcoat window when applying antifouling over epoxy primer/barrier coat
If undercoating your antifouling with an epoxy primer/barrier coat, you have to apply the antifouling paint when the primer is in the "thumbprint tacky" stage. If you miss this window, you have to abrade the surface lightly with a Scotch-Brite™ pad or 80-grit sandpaper to achieve a good mechanical bond.
Premature launching
After applying antifouling paint, always observe the paint manufacturer's specified immersion times. Don't be tempted to launch early, before the paint has cured completely. The wet paint paint will not cure properly, and your money and labor will be wasted.
Exceeding the recommended launch window after painting
After applying antifouling paint, you have only so much time to launch before the paint starts to oxidize and lose its effectiveness once immersed in the water. Antifouling paints can vary greatly (from 2 weeks to 18 months, for example) when it comes to the maximum launch window, so be sure to follow the paint manufacturer's launching specifications.
Oxidation of the surface layer of the antifouling coating
Trailered boats that are hauled and relaunched repeatedly during the season can be subject to oxidation of the antifouling coating if the bottom dries out.
If this happens and you launch the boat, the antifouling biocide can not leach properly, and biofouling organisms have an opportunity to attach. No need to repaint, but before relaunching a trailered boat on which the antifouling paint has oxidized, abrade the surface lightly with a Scotch-Brite pad to remove the oxidized surface layer. This action exposes a fresh layer of biocide that will be effective once you launch the boat.
Low salinity
Low saline levels can be due to the influx of fresh water, either from a freshwater source, such as a river, or from heavy rainfall. Even a temporary drop in saline levels can inhibit antifouling paint's ability to leach biocide. When this happens, marine fouling organisms can get a toe hold—or in the case of barnacles—an antenna hold. Once the fouling starts, it can worsen until the antifouling becomes completely ineffective.
Contaminants
Contaminants such as silt, chemicals, and other pollutants affect the pH balance of the water. High levels of acidity can actually eat the biocide in the paint, leaving nothing to leach out; high alkaline levels can trap the biocide in the paint, preventing it from leaching out. Either scenario renders the antifouling paint ineffective.
Low water level at anchorage
Boats that lie in the mud at low tide are exposed to contaminants, as described above. In addition, antifouling paint can be scraped off by hitting bottom, or by rubbing up against any sharp objects lying in the exposed, muddy surface.
Water temperature
Typically, there is more fouling in warmer or still waters than in cooler waters or in areas where the water flow is unrestricted. To help make your ablative antifouling paint as effective as possible, if you are in an area with warm and/or still waters, check the bottom regularly. Use a maroon Scotch-Brite pad to gently remove any fouling. Also, using your boat more often will help ablative antifouling paints to be most effective, because they expose fresh biocide as the boat moves through the water.
Improper grounding or stray electric current
Electric current in the water underneath and surrounding your boat can actually neutralize the antifouling paint, causing biofouling to occur. The causes of electric current are either because the electrical system on your boat is grounded improperly, or because there is stray electrical current coming from a boat anchored nearby. When you haul out at the end of the season, check to see that your electrical system is wired properly.