Varnish is a finish coating that protects your wood against sun, rain, wind, sea spray, UV rays, and chemicals. The products packaged and sold as marine or spar varnishs actually typically contain a mix of oil, solvent, resin, ultra-violet additives, and drying agents. When varnish dries, the solvent portion completely evaporates. The remaining ingredients polymerize or oxidize to form a transparent film that is extremely durable.
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.
Marine grade varnishes are engineered for life at sea. The best ones will flex with the wood, and have more UV additive content. In contrast, a typical big box store polyurethane features a higher resins ratio. The coating becomes brittle with the woods expansion and contraction due to humidity changes. As a result, the varnish does not adequately protect the wood from damage.
The highest quality spar varnishes 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 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.
Quality 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 the 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 a controlled indoor environment on very stable wood surfaces.
Spar varnish is a traditional wood coating that dries to a glossy or satin finish. From an historical perspective, the term spar varnish is derived from the spars, or masts, of sailing ships. Spar varnish enables the wood to naturally expand and contract. This varnish provides protection from water, heat, and ultraviolet light; however, it takes considerable time to cure. Choose from oil-based and water-based marine spar varnish.
Note that the term spar varnish is often used colloquially to describe any exterior grade varnish or poly finish. If you are looking for the most traditional finish possible, consider making your own varnish. 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.
In the past, oil-based varnishes were more desirable than water-based wood finishes. Traditional varnish flows on smoothly, and novices may find it easier to apply. However, newer water-based products like TotalBoat Halcyon varnish are known for its finish clarity, hardness, and durability. Oil-based polyurethanes tend to have more of the classic amber appearance, where most water-based varnishes are clearer.
Oil-based finishes generally dry more slowly, and they can easily attract dust. To clean up after oil-based varnish work, you must use soak brushes in mineral spirits or a similar solvent. Water-based finishes dry faster, tend to have lower VOC content, and make clean-up a bit easier, requiring only soap and water.
When applied per the manufacturers instructions, varnish acts as a thin film that coats the top of the wood. This film offers protection against moisture intrusion from rain and sea spray. In contrast, a wood oil penetrates deep into the wood, and cures by reacting with oxygen. Varnishes provide significantly more moisture protection than wood oils.
Your just-varnished wood will eventually fall victim to relentless sunlight, abrasion, and water assaults. As the ultraviolet light reaches the base coats, your varnish will lose its wet appearance and will eventually separate from the wood. Then, your only recourse is to strip off the old varnish and start over.
Avoid that unpleasant outcome by doing some manageable annual maintenance. Scuff and sand off the top varnish layer, and wipe the surface clean. Next, apply one or two coats to restore the glossy sheen and reinstate that valuable ultraviolet protection.
A 2-part urethane / polyurethane finish is the hardest and toughest of clear coatings on the market. Most varnishes are polyurethanes of some type, but 2-part products have a harder finish. TotalBoat Halcyon is a 1-part water-based product with a harder finish than traditional 1-part polyurethanes.
Marine spar varnish is simply the best choice for exterior use. This versatile compound has the flexibility to move with the wood as it swells and shrinks. Choose a spar varnish with ultraviolet light absorbers.
Before applying the first coat, evaluate the existing varnish. If you see varnish separation or peeling, remove the existing varnish and prepare the surface for new coat applications. Good-condition varnish may only require sanding and minor repairs.
On varnishing day, gauge the air temperature and humidity before moving forward. Low temperatures will increase drying times, while high temperatures will decrease them. A higher relative humidity will cause the varnish to dry more slowly.
For the best protection, apply a maximum of six thin coats. Use a high-quality brush and a firm stroke and tip technique. To maintain the woods appearance, lightly sand it before brushing on a new coat every year. If you neglect to do that, the varnish will eventually become brittle and crack. Then, you will have to scrape off all the coats before starting over.
Whether you choose a high-gloss, satin, or matte finish, always apply at least three full, un-thinned coats of varnish. However, many manufacturers recommend that you apply up to 10 full coats.
Most (but not all) varnishes require sanding between coats. Follow the manufacturers directions for the best quality finish.
Allow 24 hours (or more) between varnish coats, unless specified by the product. Specifically, later coats may require 48 hours to cure before you can sand them again. If you encounter a gummy spot while sanding, the varnish has not had time to properly cure. Stop sanding until that area has completely dried and cured. Then, proceed with your sanding, but remain vigilant for more gummy spots.
Sanding is a crucial step in the varnishing process. When completing your pre-varnish surface prep work, sanding evens out the high spots so you will varnish a flat surface and achieve a better finish.
Sanding between varnish coats gives the wood a slightly rougher texture, and enables the next coat to adhere better. Finally, sanding between coats makes it easier to see the holidays, or spots you may have missed. Using an off-center light source will help in that regard.
Yes, you may be able to apply new varnish over the existing varnish. First, de-contaminate the surface with a manufacturer-recommended thinner. If the existing varnish is in good condition, a light sanding and wipe-down with a manufacturer-directed compound sets the stage for a new coat.
If the varnish is in good shape with some damage, repair the issues with a wood filler stain. Then, spot prime and varnish those surfaces before sanding again. Finally, apply a full varnish layer.
For poor-condition varnish with cracking and peeling, you will waste your time by brushing on another layer. Instead, remove all the varnish by sanding, scraping, or using a chemical paint stripper. Then, prepare to begin the entire process again.
First, always varnish with a high-quality brush. However, a roller may be more time-effective if you are varnishing a large flat surface. To brush the varnish onto the wood, use a firm stroke along and across the woods grain. Hold the brush at 90 degrees to the woods surface.
Next, tip the varnish by lightly stroking the woods surface while holding the brush at 45 degrees. To avoid contamination (and inferior results), do not use your varnish brush for any other purpose.
Yes, you can spray marine varnish, and quality tools are key to the best result. Buy a quality spray gun, along with an air compressor that delivers sufficient cubic feet per minute to the spray gun. A Zahn Cup will enable you to consistently measure the thinned varnishs viscosity. A respirator (not a dust mask) will help to protect your lungs. To thin the varnish, purchase the manufacturers recommended solvent.
Spraying is an advanced application method that takes considerable practice. You can achieve great results by properly applying the varnish with a high-quality brush.