Marine Varnish has long been used as a protective and preventative agent for protecting wood in a wet, marine environment. Spar varnish formulations for use on boats are typically a mix of oil, resin, solvent, dryers and ultra-violet additives. When combined, these ingredients serve to keep wood at optimum functionality. Different oils increase the penetration capabilities of the varnish, while solvents and dryers treat and expel excess moisture. Resins, both organic and synthetic, allow wood to dry faster, with better water resistance. Finally, ultra-violet additives protect wood from damaging UV radiation from the sun, prolonging the life and luster of the wood. By varying these components, and by adding UV inhibitors and other secret ingredients, each particular varnish will have different characteristics. Varnish can have gloss or matte appearance, can be extremely hard and durable, and can vary in color. The highest quality ones presently use China Wood Oil, which is more commonly referred to as Tung Oil. Derived from trees, China Wood Oil provides long-term resistance to cracking and crazing. Soya oil is mostly found in standard or economical choices
The main purpose of oil in a varnish is to improve penetration into the wood. The more oil in a varnish, the better the penetration. China Wood Oil has been maximized for this purpose. Some manufacturers add Penetrol to to enhance its penetration characteristics. Interlux does not recommend this is because it may have an adverse effect on the longevity of the varnish. When discussing oil, the terminology long, medium, and short oil is sometimes used. This refers to the ratio of oil to resin in a particular coating. The long oils tend to result in longer dry times but greater durability in terms of gloss and color retention. Premium varnishes exhibit these qualities. Medium oils allow for faster drying times. They are, generally, restricted to low-grade varnishes. Short oils are used almost exclusively on primers.
Choosing The Right Varnish
Quality marine varnishes fall into 2 basic categories, modern hard varnishes and traditional spar varnishes. The choice boils down to two main considerations: How stable is the wood you are coating? If it is prone to movement, such as a spar, old clinker, or carvel planked boat, traditional varnish is the right choice. If the piece is more stable and inert, a harder modern finish is also an option.
While modern finishes resist wear better, they are also harder to remove when you recoat. Some require strict temperatures and conditions for application that may not suit a boat out in the open. Traditional marine varnishes apply easily but typically have a longer cure time. Higher tung oil content provides a beautiful golden hue and that almost-wet appearing gloss. Traditional varnish builds on top of wood surface with minimal penetration, making it easiest to remove when the time comes.
Monourethanes & single polyurethanes combine the best of old and new: they yield a harder finish that cures quicker via moisture content in the wood. The finish can be polished and is easier to remove than a two-part. Cure time is often optimized to allow several applications in one day. These hybrids also contain oils to create a more pliable coating with the traditional look.
Two-part polyurethanes and epoxy varnishes produce the most durable, scratch resistant finish available and done properly yield long lasting beauty. Application is more challenging, as they require specific temperatures and cannot be exposed to rain or dew during the curing window. These coatings penetrate into the wood, creating a superior coating capable of lasting several seasons but require sanding to remove. This is worth considering if it's a classic boat with irreplaceable woodwork. Best used in controlled indoor environment on very stable wood surfaces.
Key to achieving a professional looking finish is a high quality varnish brush, with natural bristles like badger hair. A badger hair brush has thick bristles to load more varnish. Since a good varnish will self-level, or flow out evenly across the surface, this thicker brush lets the varnish level out as designed. The key is to start from one section and maintain the wet edge without going back over the work. You will achieve better coverage with no visible brush strokes. If you are applying a modern clear coating treatment such as a 2-part poly, a thinner brush is preferable. Before applying, be sure to shake out any loose bristles by gently and rapidly wiping across a clean hand. Another option for quick, no cleanup jobs are disposable foam brushes. They are cheap, convenient and will not leave bristles behind. Foam brushes don't leave behind brush strokes, a plus if you are inexperienced.
Take all of this into consideration when you are choosing the final, finishing varnish for the wood on your boat. Wood has a natural beauty that many people wish to not only protect but to enhance. Varnish provides a barrier to protect against the elements and things that will attack wood such as sea, rain, wind and sun.
If you want to go the really traditional route, make your own! Our JD Homemade Varnish Recipe is an old Down East deck coating formula traditionally used on wooden decks for schooners, fishing boats, and porch decks. You will need 1 qt of Turpentine, 1 qt Boiled Linseed Oil, 1/2 pt Pine Tar, and 1/2 pt Japan Drier. What results is a darker, amber finish. To customize the mixture, add more pine tar for a darker color or add less for a lighter color. Allow more drying time for the darker mixture.