Varnish vs. Polyurethane

Here is a great article TinBoats member Johnny put together to highlight the difference between Varnish vs. Polyurethane. If you have any questions for Johnny, feel free to enter comments below, or better yet reach out to him on our forum.

Before we dig into the details, let’s talk about why we need a different varnish for outdoor applications. Any wood that is used outside is going to be exposed to a wide range of climate conditions, as well as a good dose of damaging UV rays. These elements serve to break down the finish over time. Furthermore, changes in humidity cause the wood to expand and contract, and a standard indoor finish would simply crack and deteriorate under these conditions. Spar varnishes are typically designed to not only protect the wood, but also give it the flexibility and UV protection it needs to last for years. And the name “spar” varnish comes from the boating world, where, on a sailboat, the long wooden poles that support the sails are known as spars. So a spar varnish needs to be one that can withstand the rigorous conditions of the lake and ocean seafaring life.


Polyurethane is like a liquid plastic. Often either a pure synthetic solid plastic or a blend with resin in the liquid form. There’s an option of a water or oil based resin as well as sheens from flat to satin to glossy. Despite its sometimes milky appearance in the can, polyurethane goes on clear and in just one or two coats.
It cures into a scratch- and abrasion-proof hard plastic that is versatile enough for most indoor projects, especially wood floors.
Most polyurethane finishes do not have sufficient UV blockers in their
formula for long term outside use and should be avoided in such circumstances.


Polyurethane and traditional varnish are two popular finishes that cure into durable protective coats when applied to wood. But, although they’re often referred to interchangeably, each one has its own distinct uses and offers varying levels of protection from the environmental elements. To learn which product is best suited for your next project, careful attention must be directed to the manufacturers “list of ingredients”. In order to attain the perfect protection for your project, you must use the correct finish that will do the best job correctly and satisfy your expectations.


Nearly all modern varnishes contain a few basic components:
oil, resin, and a solvent.
Oils – Linseed Oil, Boiled Linseed Oil or Tung Oil
Resins – Alkyd, Phenolic, or Polyurethane
Solvents – Mineral Spirits, Naphtha, or Turpentine. (pure turps, not synthetic).
By modifying the types and amounts of these basic components, you can create a whole range of mixtures that vary in application requirements.

Oil to Resin Ratio
When a varnish is formulated, the ratio of oil to resin can have a dramatic effect on the way the varnish will behave. For instance, using a small amount of oil and a large amount of resin will produce a very hard but somewhat brittle finish. Obviously, this is not suitable for outdoor applications since we need an outdoor finish to be flexible. So what makes more sense is to create what is known as a “long-oil” varnish. It is a formulation that contains a greater percentage of oil. The extra oil results in a softer, more flexible finish that will not crack when the wood expands and contracts.

Oil Types
The most common oil used to make varnish is Linseed Oil. Its lower cost makes it the most practical choice for both indoor and outdoor formulations. But many believe that Tung Oil is actually better for outdoor use. After all, a higher quality oil should equate to a higher quality of varnish, and thus a higher price tag.
As a result, many of the high-end marine varnishes will be made with Tung Oil instead of Linseed Oil or Boiled Linseed Oil. Tung Oil is made from pressed seeds from the nut of the tung tree. The tung tree is native to China. Pure Tung oil is considered a drying oil much like linseed, safflower, poppy and soybean oil and is known to have a slightly golden tint.

Tung oil

Raw Linseed Oil vs. Boiled Linseed Oil:
“Raw” linseed oil is just that – linseed oil is squeezed from flax seed and packaged with no additional additives or preservatives. Raw linseed oil dries very slowly, taking several weeks to fully cure. You should limit its use to items exposed to the elements where drying time is not a factor.“Boiled” linseed oil is not really “boiled” in the since of heating it. The term comes from the addition of drying agents which promote a faster drying time. When raw or boiled linseed oil is added to an oil based paint or varnish, you change the drying time, penetration ability and finish accordingly.

WARNING: Any rags used to apply coatings, oils, stains or solvents should be thoroughly air dried outside prior to storing or discarding in the trash.

Resin Types
Generally speaking, phenolic resins are best-suited for outdoor use. But that doesn’t mean every “spar” varnish is made with phenolic resins. Much like the situation with oils, the better product is also the most expensive. So you’ll find plenty of outdoor formulations using alkyd and urethane resins. A popular finish like Helmsman Spar Urethane contains urethane modified alkyd resins. A higher quality finish like Epifanes contains phenolic modified alkyd resins with no urethane. The UV inhibitors are separate ingredients and added after the final mix. There are many brands of outdoor oil-based varnish. But the actual ingredients list is usually much more revealing than the attractive words on the front of the can. Read and Understand the formula and suggested uses for the product. Phenolic resins are divided into two different types, novolacs and resoles. Both have high temperature stability up to 300° – 350°f. Also, phenolic resins are quite stable when exposed to water and chemicals. Phenolic resins are often dark-colored from yellow to dark red, and have an excellent price/performance profile and are found in a myriad of industrial products. They are mainly used in the production of circuit boards and molded products including billiard balls, laboratory countertops, and as coatings and adhesives in severe elements.

UV Blocker and Sealer For Wood
Most spar marine varnishes will contain other important additives, such as UV blockers, absorbers or inhibitors that give the wood that extra bit of protection it needs in harsh conditions. UV light will not only damage the finish, but also the wood itself, eventually resulting in finish failure and possible wood deterioration.
So, its a good idea to use a finish containing high levels of UV-blockers for any and all outdoor projects. Especially if used around the water such as lakes, oceans and beaches.

Thinning and Modification
The term “thinning” is to reduce the viscosity of the varnish or paint which allows the first layer to penetrate well into the wood rather than just lay on top and dry.
This improves the longevity of the coating system. Keeping moisture out of the layers beneath the surface. The preferred thinner would be Mineral Spirits, Naphtha or Turpentine. Allow to dry and cure thoroughly between coats as suggested in the manufacturers application instructions. The type of thinner and the amount is determined by the climatic conditions of where it is initially applied. In a climate controlled workshop or garage could be much different than out in the open hot, cold, sunny or cloudy environment. The basic differences in the thinners mentioned is this . . . .
Turpentine: a bit on the slow drying side, a retarder. Used in hot weather.
Mineral Spirits: the favorite among painters and wood finishers. It thins well and does not inhibit preferred drying times.
Naphtha: (pronounced Naf-Tha) a fast drying thinner. Used in cold climates or when a quick drying finish is required. NOT to be used in a penetrating sealer
as it sets up too quickly and prevents adequate penetration into the wood fibers. Once a thinner is added to the mix, it is then called “modified”. DO NOT return any modified product back to the original container or else it will spoil the whole lot.

Durable Showroom Finish: In order to attain the showroom finish that is coveted in the wooden boat arena, such as Chris-Craft, one must commit to the labor intensive maintenance program that this type of finish requires. With this dedicated practice, the beautiful varnished finish will last for years. With the appropriate thinner, dilute the first coat 50%, the second coat 25%, the third coat 15%, and any additional coats anywhere from 0 to 7%. [If you used a wood sealer, thin the first coat 25%.] Do not thin your last one or two coats. For the system to work properly, it will take a total of 12 to 14 coats to establish the base coat for depth of gloss and adequate UV blocking. Expert studies have proven that you need at least 12 coats to get good UV protection. If you try to get by with fewer coats, you may have lifting and flaking after one summer in the sun. Then, you must sand it down and start all over again. Every year or two, abrade the finish lightly and apply two thin coats to maintain the ultra-violet protection, as this UV inhibitor is the sacrificial layer in the varnish and is used up by exposure to the sun. So it must be replenished frequently. Much like applying sun block to your skin to protect you from solar radiation. It needs to be reapplied periodically to maintain the effectiveness of the UV blocker to protect you from sunburn.

Household Spar Varnishes typically have very little “true” UV inhibitors.
The frugal manufacturers will typically just have a few drops of the UV inhibitors in the formula to comply with the truth in advertising regulations. So their resistance to the harsh UV elements is not much better than regular furniture grade varnish or polyurethane.

Marine Varnishes, which are far more expensive, do contain considerable amounts of UV inhibitors. So if you need full UV protection, it would be in your best interest to seek out the name brands that have the specified ingredients that your project requires. And, be prepared to pay much more for the better materials.
Bottom line is – you get what you pay for.

Drying Accelerator
Japan Drier is a common drying agent that can be mixed with other oils such as boiled linseed oil and alkyd resin paints and varnishes to speed up the “drying” or “curing” process. Japan Drier is used by professionals to speed up controlled drying when they need to apply several coats on the same a day. Japan Drier is a special blend of lead-free drying agents that accelerates the drying ability of oil-based paint, enamels, varnish and polyurethane. It is especially effective in highly humid or cool weather conditions. Although Japan Drier may appear purple, it will not affect the color of your coating. (varnish or paint). Normally, one ounce of Japan Dryer per quart of paint or varnish and mix thoroughly. Do not exceed 4 ounces per gallon. Use the minimum recommended amount in white paint since discoloration may occur in higher concentrations.

Plywood is an extremely porous wood product. If not adequately sealed, moisture intrusion will greatly shorten the life expectancy. Whatever product is used to seal the plywood, ensure it is thin enough to penetrate deeply into the plys and wood fibers and allowed to cure properly before applying the subsequent coatings.

Penetrating Sealer (waterproofing) for plywood transoms and boat floors. As mentioned in this article, “one size does not fit all”.With experience and knowledge, you can adjust the blend ratio of the varnish and paint with thinners to suit your needs. Again, once an additive is added to the mix, it is then called “modified”. DO NOT return any modified product back to the original can or else it will spoil the whole can. (Which could be costly in the higher priced coatings).
One of many blends of a home-made penetrating waterproof sealer is to add the following products together:
1 part varnish
1 part boiled linseed or tung oil
2 parts mineral spirits. Don’t use naphtha. (it dries too fast).
One note of interest: If you are in a really humid area where mildew is frequent, skip the addition of linseed or tung oil and use a straight mix of 50/50 plain mineral spirits and your preferred varnish for the initial penetrating sealer. Additional coats of the undiluted product will give additional protection.
The reason for omitting the oil is that the long oils actually “may” promote mildew and mold within the wood. (not a good thing).
Adjust the mix to suit your application and project requirements.
Apply liberally with brush, roller, spray or rag. When the surface appears to be saturated, wipe off the excess. allow the project to dry for a minimum of 48 hours before applying additional coats. Two or more coats of straight varnish applied 24 hours apart will seal the wood quite well. But still, it must be maintained.
Varying climatic conditions and time of day can affect the drying/curing process.

WARNING: Any rags used to apply coatings, oils, stains or solvents should be thoroughly air dried prior to storing or discarding in the trash.

Generally speaking, the preferred outdoor varnish would be a long-oil varnish made with tung oil and at least some phenolic resins and ample amounts of UV inhibitors. (name brands are not mentioned in this article as the person doing the application should compare the ingredients of each manufacturer’s product to suit his or her project requirements).

The amount of varnishes and paints used in today’s world are often overwhelming to the average person. If you are restoring a 1940 Chris-Craft mahogany runabout, you would want to use the highest quality materials available, regardless of cost. If you are just replacing or refinishing a plywood transom, wood seats or floors on a Jon Boat, you really don’t need to spend $65.00 for a quart of paint or varnish when the more inexpensive coatings will do the job just as well. (but, not for long. It will require annual maintenance to maintain the desired protection). Proper preparation before applying any coating will result in a more durable and favorable finish.
Note: the generic term of “VARNISH” in this article would include the same
chemical makeup of different solvent/oil based varnishes, paints and coatings. NOT water based varnish, polyurethanes or coatings.

READ, UNDERSTAND and FOLLOW the instructions on the label  of any and all products you may use. Observe all safety precautions !!!

Sources of Information: Please make sure to check out the links!
DIY Wood
Popular Woodworking
Fine Woodworking
The Wood Whisperer
Wooden Boat
Classic Boats
As well as my own personal experience of 50 years in the sign painting and custom wood manufacturing industry.

Johnny – 2016

One thought on “Varnish vs. Polyurethane

Leave a Reply

Your email address will not be published. Required fields are marked *