Anti-fouling paint protects the bottom of a boat’s hull from slime, weed, and barnacles. The active ingredient in the paint matrix is a biocide which differs depending on the location factors (water flow, salt concentration, temperature) and the type of usage. The three major types are (1) traditional, erodible, and hard paints. This article covers the trade-offs with each and their unique maintenance issues.
Per the name, traditional paints are outdated and not common. The matrix includes a biocide (usually cuprous oxide) which is suspended in a hexagonal resin structure. The biocide leaches out to fight against foulants and when exhausted leaves a honeycomb residue. This residue builds up over a few years and must be wet abraded. To switch from traditional paint to one of the other types, re-seal the bottom with a coat of underwater primer.
Erodible paints are the common paint these days and work through ablation of the paint matrix. The result is less build-up than with traditional paint. Instead of the biocide leaching out, the whole bottom wears in a controlled way. This type works well for yachts traveling at less than 25 knots but is not viable for motor cruisers who routinely reach higher speeds for long durations. To switch from erodible paint, completely remove (via exhaustion or wet abrasion) the matrix before applying one of the other types of paints.
Hard paint is composed of a matrix of insoluble resin. The soluble biocide acts via contact leaching. As each biocide particle dissolves, the matrix is structured so that an adjacent particle becomes exposed. This type is resistant to abrasion, so it is ideal for high speed power yachts and yachts that sit in the mud at low tide. It is superior for racing yachts because of the smooth, low friction finish. To overlay, wet abrade the surface. A special type of hard paint is Teflon paint. It has a slippery finish that is about friction free, resistant to damage, and easily cleaned. To refinish, do not abrade – just wash and repaint.
The three types of anti-fouling paint I outlined are traditional, erodible, and hard. Traditional is outdated; erodible makes the most sense for cruisers; high speed or racing yachts prefer hard paints. Common to all types are many other issues such as the yacht location and the usage. Application procedure is a final commonality (number of coats, masking, launching timeline). There are quite a few details inside what seems like a simple topic – painting the bottom of your boat.