Surface treatment bundling and lacquering of hull

The surface of a boat is more than cosmetics. The gelcoat the lacquer and the bottom paint are the hull's first and most important barrier against water sun and the organisms that would like to settle on an underwater surface. If you understand how the different layers work together you can both prevent damage and fix it when it occurs anyway.

§Gelcoat and lacquer — the first barrier

On a fiberglass hull, gelcoat is the outermost, colored resin layer applied to the mold before the laminate is cast behind. It gives the hull its color and shine and is the first protection against water and UV light. On a wooden boat, varnish or oil plays the same role: it seals the wood's surface and protects against moisture and sunlight, which otherwise break down the wood's fibers over time. Both must be maintained — a scratched gelcoat or a worn varnish is an open invitation for moisture to penetrate deeper.

§When water penetrates—osmosis

One of the most known problems on older fibreglass boats is osmosis also called fiberglass blight. It appears as small blisters under the gelcoat typically below the waterline. The cause is that the fibreglass laminate contains small air bubbles dust or residues of not fully cured polyester from the very casting. Over time water vapour penetrates through the gelcoat and any primer and collects as water in these cavities where it via osmotic pressure draws more water to it and forms the blisters.

§Bottom paint and fouling

Below the waterline bottom paint (antifouling) is used to keep algae sea squirts and other organisms from attaching which otherwise increases water resistance and can hide signs of damage. Bottom paint contains biocides and because these substances can harm the marine environment products must be approved before they can be sold and used in Denmark. There are both 'hard' bottom paints which remain and must be cleaned or polished between seasons and 'soft' or ablative bottom paints which gradually wear off so a thin active layer is constantly exposed.

Steps in typical primer coating process

§Paint and oil for wood

On a wooden boat, varnish and oil are not just decoration. Clear varnish in many thin coats protects freeboard and deck from UV light, which otherwise breaks down the wood's surface and makes it gray and brittle. Teak on deck is often oiled instead of varnished because it withstands wear better, while freeboard is typically varnished for shine and protection. What is common to both is that moisture control and good sanding between coats are crucial for a durable result.

§Environmental requirements for base paint

Because bottom paint is made to actively kill organisms it is subject to rules about which substances may be used. Internationally particularly harmful substances such as tributyltin (TBT) have been phased out via IMO's AFS convention and locally products must be approved before they can be marketed. As a craftsperson it is therefore part of the trade to know and use the approved products correctly — both for the benefit of the marine environment and to avoid liability for an illegal product.

“The surface is the hull's skin — it must breathe in the right place close tightly in other places and it feels first of all if something underneath has started to fail.”