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Marine Paints and Varnishes - Antifouling - Fiberglass Osmosis

Sections: Antifouling Preparation and Painting Fiberglass Osmosis Blisters

Best antifouling paint for fiberglass anti fouling paint marine polyurethane varnish sailboat maintenance

To preserve the structure and ensure the watertight integrity of wooden hulls, tree resins, natural tar, animal fats and vegetable oils were all used by the early boat builders, and the addition of pigments to these materials gave the colours encouraging owners to decorate their vessels.

Nowadays there are three types of marine paint protective coatings:

  • oil-based paints and coatings which dry by the evaporation of some of the oil;
  • polyurethane, which include varnishes and cure when in contact with the air;
  • and two-part epoxies which cure by chemical reactions between the two parts.

To bring out the natural colour of all wooden surfaces, including the on-deck varnished woodwork or 'bright work', varnishes were produced without pigments mainly used on the darker varnished mahogany of cabin sides, hatches and cockpit. Teak structures, such as grab rails, are treated with teak oil penetrating the wood and preserving its natural oiliness.

Wooden boat interiors of were often finished with contrasting white planking and varnished beams and furniture with matt finishes often giving a better contrast than the bright finishes which show up areas of wear.

The bilge area is usually treated with coal tar derivatives, or bilge marine paints containing fungicides, to prevent the onset of dry rot. The bilge is a difficult area to keep clean, and is a repository for stagnant water and engine oil spillages.

The topsides of the boat demanded finishes which could be applied evenly and would dry to a perfect, hard gloss finish. White has been a favourite but is one of the most difficult to apply because it shows up blemishes, especially older GPP hulls requiring painting to prevent the ingress of water into the laminate through a deteriorating gel coat.

Marine Antifouling Paint

The boot topping is the area starting at the junction between sea and air and three inches above the waterline on the hull. Boot top paints require a mixture of the harder topside paints and coatings and the softer, leaching anti-fouling marine paint and most owners use a contrasting colour for boot tops.

The battle to keep the hull and the elements apart is below the waterline, where fouling resulting from the growth of weed and barnacles results in loss of performance. Tropical waters have harmful marine borers which penetrate wooden hulls and decimate the wooden structure.

Copper sheathing and copper and arsenic-impregnated antifouling paints and coatings were successful in combating growth and borers, but hulls still needed periodic scrubbing.

The introduction of tributyl tin (TBT) in the 1970s were excellent antifouling agents, but their harmful effects on marine life resulted in a complete ban on their use in the late 1980s and a return to the copper and arsenic based antifouling paints.

Two grades of marine antifouling paints are produced:

  • The hard racing finishes which can be sanded smooth and leach slowly,
  • The softer cruising versions which form a thicker protective coating which leaches quickly to prevent weed and barnacles from gaining a foothold.

Preparation and Painting

The most skilled boat maintenance tasks is the application of marine paint and requires considerable time, patience and perfect working conditions. Successful finishes rely upon particular preparation of the surface to be protected or decorated, along with surfaces that must be filled and sanded so that all blemishes are removed on unpainted wood and Glass Reinforced Polyester (GRP) surfaces.

This surface has to be cleaned several times with degreasers and thinners, to ensure that the first phase of the paint system can key into the porous structure and the whole finish does not flake off. Throughout the application process each successive must be sanded down with and-dry paper and cleaned with a ‘tack rag' to remove all traces of dust so that the many layers of marine paint can be applied.

Marine paint systems start with a primer, acting as a keying agent and filler and sometimes the primer is combined with an undercoat. After priming and undercoating, a number of top coats are applied to give the desired finish and brushes, rollers and mohair pads used to assist in producing the result.

There is a need to balance the quick-drying properties of the product along with a need to prevent runs and to preserve a 'wet edge' that allows successive strips of painted boat to merge into one smooth skin free of brush marks.

Glass Reinforced Polyester (GRP) hull treatment is best left to the professional spray shop where two part epoxies to replace the original gel coat finish with an impervious coating, require a certain amount of expertise.

Fiberglass Osmosis Gelcoat Blisters

Treating the undersides of the hull to remove gelcoat blistering caused by fiberglass osmosis requires the removal of the entire gelcoat by grit blasting or sanding. Fiberglass osmosis is the slow seepage of water into the laminate causing leaching of the resins showing up as gelcoat blisters. The exposed laminate must be dried before a new epoxy barrier is built up and sanded to a perfect finish. This process is best left to the professionals.

Steel and Aluminium Hulls

Steel and aluminium hulls require treatment with marine paint systems particular to their individual characteristics. The primer is an inhibitor to prevent the formation of rust or corrosion, thereafter, the paint is designed to build up an elastic protective layer to prevent ingress of salt water getting at the metal.

The wide variety of finishes available must be researched thoroughly before purchase. If you require a traditional but renewable finish, traditional marine paints and varnishes are the solution. Polyurethanes give quicker results but varnishes can peel off if moisture gets into the wood. Clear epoxy finishes may need a final protective coat of ultraviolet light stabilizer to prevent them deteriorating after prolonged exposure to sunlight.

removing antifouling paint from fiberglass