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Wood Finishing #1: Varnish vs Polyurethane

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Blog entry by John Smith posted 09-20-2019 03:48 PM 592 reads 3 times favorited 3 comments Add to Favorites Watch
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Varnish vs. Polyurethane

Before we dig into the details, let’s talk about why we need different finishes 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 (such as polyurethane) would simply crack and deteriorate under these conditions. And this could result in a whole host of unnecessary repairs.

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 (masts) 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.

What is Polyurethane:
Polyurethane is like liquid plastic. Often either a pure synthetic solid plastic or a blend with resins in the liquid form. There is an option of 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 and is generally brushed or sprayed onto the project.
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.

What is Spar Varnish:
Traditional varnish and polyurethanes are two popular finishes that cure into durable protective finishes when applied to wood. But, although they are often referred to interchangeably, each one has its own distinct use 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 long term expectations.

Varnish:
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 – Pure Mineral Spirits, Naphtha, or Gum Turpentine. (100% pure, 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 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 resist cracking and separation 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:
Tung Oil is made from pressed seeds from the nut of the tung tree which 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.

Linseed 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 months to fully “cure“. You should limit its use to items exposed to the elements where drying time is not a factor. “Boiled” linseed oil (a/k/a BLO) is not really “boiled” in the sense of heating it to its boiling point and cooking it. The term comes from the addition of drying agents which promote a faster dry and cure 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 phenolic, 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 product’s label. 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 Blockers, Inhibitors and Sealers:
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. The UV light will not only damage the finish, but also the wood itself, eventually resulting in finish failure which can lead to wood deterioration.
So, it’s 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.
Note: (I could not find one company that actually told how much of the UV blockers are in their product.
not one. so it is strictly on the shoulders of the consumer as to which hype to believe – one over the other).

Thinning and Modification:
The term “thinning” is to reduce the viscosity of the product which allows the first application to penetrate well into the wood and create a foundation for subsequent coats rather than just lay on top of the wood and dry.
This improves the longevity of the coating system. Keeping moisture out of the layers beneath the surface. The preferred thinners would be 100% Mineral Spirits, Naphtha or Pure Gum Turpentine. Allow to dry and cure thoroughly between coats as suggested in the manufacturer’s application instructions. The type of thinner and the amount is determined by the climatic conditions of where it is initially applied and the method of application.
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.
Gum Turpentine is obtained by the distillation of sap tapped from living pine trees. Turpentine has more solvency than mineral spirits. Its high solvent strength makes it the best choice for thinning oils and natural resins.
Gum Turpentine also is a natural antiseptic that will deter mildew and mold in organic substances, such as wood.
Mineral Spirits: (a/k/a Paint Thinner) 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 could 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 15 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, the finish may have cracking, lifting and flaking after just one summer in the sun. Then, you must sand it down and start all over again. Every year or two, clean the project thoroughly to remove any possible contaminants, 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.

A good Marine Spar Varnish is much like applying sunblock lotion 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/Urethane 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 interior 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 better materials. And apply them as per the manufacturer’s instructions.
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. It’s also used by professionals to speed up controlled drying when they need to apply several coats on the same day. Japan Drier is a special blend of lead-free drying agents that accelerate 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 (1:32 ratio) 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 plies and wood fibers and allowed to cure properly before applying the subsequent coatings. Even Marine Grade plywood will deteriorate if not properly treated and maintained on a regular schedule.

Penetrating Sealer:
(waterproofing) for plywood boat seats, transoms, floors and other porous woods.
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 container or else it could 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 oil varnish
1 part tung oil
2 parts mineral spirits or turpentine. 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 Boiled Linseed Oil (BLO) and Tung Oil and use a straight mix of 50/50 pure Gum Turpentine and your preferred varnish or paint for the initial penetrating primer or sealer. Additional coats of the undiluted product will give additional protection.
The reason for omitting the BLO and Tung Oil is that the long oils actually “may” promote mildew and mold within the wood itself which could lead to premature failure. (not a good thing). And the turpentine has the natural antiseptic qualities that mineral spirits and other thinners do not have.
Adjust the mix to suit your project requirements and application methods.
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 (or paint) applied 24 hours apart will seal the wood quite well.
But still, it must be maintained on a regular schedule to have long lasting satisfactory results.
Varying climatic conditions, time of day and application methods can affect the drying/curing process.

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.

Conclusion:
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 number 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 $85.00 for a quart of paint or varnish when the more less expensive 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 satisfactory 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 to be confused with water based varnish, polyurethanes or other water based 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:
DIY Wood Boat.com
Popular Woodworking
Fine Woodworking
Wood Finishing w/Bob Flexner
Brightwork w/Rebecca Wittman
The Wood Whisperer
Wooden Boat
Classic Boats
As well as my own personal experiences.

John Smith – 2016

-- I am a painter. That's what I do. I paint things --



3 comments so far

View Blackfin29's profile

Blackfin29

125 posts in 679 days


#1 posted 09-21-2019 03:20 AM

GREAT explanation..

Only problem is … WTF finish do I use??? hahha… I believe the only way is to use it, and test it to see if it lasts.

View wildwoodbybrianjohns's profile

wildwoodbybrianjohns

306 posts in 57 days


#2 posted 09-21-2019 04:22 PM

Great article John, thanks for that. I actually learned something.

-- Wildwood by Brian Johns: It is wiser to find out, than to suppose (S. Clemens)

View ocean's profile

ocean

179 posts in 1343 days


#3 posted 09-24-2019 08:56 PM

Great brake down of all the ingredients used in modern finishes. As you so well put – read the ingredients in each product will show you why it is a cheap or expensive product. Then use the one that fits your budget as well is the best for your project. Thanks.

-- Bob, FL Keys

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