Wipe-On Varnishes and Oils (Page 1 of 2)
Finishing Secrets No. 7 (Revised 8/23/2009)
A wipe-on oil with varnish resins is an ideal finish for the woodturner. It is easy to apply, very forgiving, easy to repair, and gives the wood a durable protective film that is not easily damaged by water, food acids, body oils, or solvents. The only negative to using varnish as a wipe-on finish is the time that is required to build a high gloss surface film, and the “blotchy” discoloration that is often associated with oil finishes.
Discoloration, darkening and yellowing with age are the result of the resins and oils that are used. However, this can be corrected to a large degree through selection of the varnish resins and oils, and sealing the wood to limit the penetration by the oils.
In their simplest form, the wipe-on varnish is an oil, either Tung Oil, Linseed, or some other "drying oil", to which varnish resins have been added. Without these additional resins, it is called "Danish Oil". These resins are harder than the polymerized oil, and impart a harder more durable surface film on the wood than the Danish Oil without them. Most of these finishes now use Polyurethane resins, and the popular "poly finish" is nothing more than a simple Danish Oil finish to which polyurethane resins have been added.
Other resins such as alkyd, phenolic, and any number of natural resins and gums, may also be used. These other resins are more expensive than the polyurethane, however, they may have some properties that we will like better.
Reading the resin content of a finish from the label can be confusing.
For this discussion, I am using the term "resin" to define those that are ADDED TO the oil in the product and not the hardened oil itself.
The Oils
Unless it is a waterborne finish, the oil is the carrier of the resin. Which oil is used in a commercial finish is determined by the targeted price on the store shelf. Drying Oils can be used straight, but are often accelerated with oxidizers to make them cure faster. Many commerfial finishes use non-drying oils because they are less expensive, and then add odidizers in the form of drying agents to force them to cure.
As a straight oil finish by itself, Tung Oil develops a hard surface that is waterproof, doesn’t darken or turn yellow with age, and doesn’t discolor the wood. But, it doesn’t penetrate the surface very well, it takes forever to dry, and is more expensive than other oils. Partially polymerized Tung Oils solve the drying problem, and thinners can improve its penetration. However, these products are expensive and can cost upwards of $30.00 for a quart.
Linseed, soybean, and other vegetable oils require a drying agent, but they offer faster drying and better penetration, but they also turn very dark with age, many become rancid, they are not as durable, and they must be periodically renewed. They are also are far less expensive than Tung Oil.
The Varnishes (a very brief summary)
For our purposes, and without going into the chemistry, there are five (5) kinds of varnishes available to us on the store shelves. All of them are in an oil carrier, and which oil determines the price. They are -
- alkyd resins- phenolic resins- polyurethane resins- spar- quick-drying Alkyds are the traditional varnishes of years past, and what you will find on furniture made before 1960. They are durable, flexible, resistant to abrasion, have good adhesion qualities, resistant to discoloration from UV and light, and are relatively less expensive than the other varnishes. Their only negative is that they are very slow drying and will take 24-hours or longer to dry sufficiently for recoating. For this reason alone, the alkyd varnishes are becoming difficult to find as a furniture finish, but their superior UV qualities make them the choice for floor finishing varnishes. The last alkyd varnish I saw was the ValSpar brand that is sold at Lowe's, but even that has disappeared from their shelves in recent years.
Phenolic resins have been around since there has been plastic. A crude comparison would liken phenolic resin to Bakelite®, waterproof, and very hard. They are still available in Spar Varnishes, and in the Waterlox commercial finishes.
Polyurethane resin varnishes first came on the market in the late 1950's (my memory of them). Those first finishes were terrible, but they were advertiased as new and better. They have improved somewhat over the years, and they have all but replaced the traditional resins.
The poly varnishes have replaced the alkyds and phenolics resins for three (3) reasons that have nothing to do with the finish on the wood – they cure faster, they are less expensive, and they have been advertised being superior because they are a result of "better living through chemistry".
Several of the benefits of the alkyds and phenolics have been sacrificed. Polyurethane varnishes are not light and UV resistant, and therefore will turn very “yellow” rather quickly. They can become very hard and brittle with age and exposure to sunlight. Refinishing is difficult at best. They can also have adhesion problems with themselves and other finishes. Poly varnishes have improved over the years, and there are some that have only partially solved these problem; but none have solved them sufficiently to make them better than the alkyd and phenolic resin finishes. Their only superior property is that they are more water resistant.
A true Spar varnish is a formulation of phenolic and alkyd resins in Tung Oil. . Spar varnish is a relatively hard finish with superior water resistance and flexibility. It has a good resistance to damage from either acid or alkali substances or deterioration from light and UV. Unfortunately, it is very slow drying and it has a naturally deep yellow color.
The spar varnish I prefer is the McCloskey's in the red can. It is available from many paint stores, and I buy it at Wal-Mart.
Many Spar Varnishes now use polyurethane resins, and the "Helmsman" brand by Minwax is the best example. This is the only poly-varnish that I will use for finishing turned wood.
Fast-drying VT varnishes have been modified with styrene resins to produce a very fast drying time that is similar to that of a nitrocellulose lacquer. This speed comes with the sacrifice of the protective properties found in the other varnishes. I am mentioning them here because they are a good varnish for finishing turned wood.
My preference for a VT varnish is the one by Sherwin-Williams.
Which Varnish?
It should be concluded that the properties of either an alkyd or spar varnish are best suited for our woodturnings. Either will offer the flexibility to move with the wood as it changes with seasonal moisture changes, and provide the durable surface protection required for an item that will see frequent handling over a period of many years. Although I have preferred the alkyd varnishes for many years, it has become difficult to find them on the store shelves. However, alkyds are readily available as a floor finish, but usually in nothing less than a 1-gallon container.
I started using spar varnish several years ago because it was easier to find. I have not found it to be superior to the alkyd as a finish for turned wood.
Many woodturners prefer the higher gloss from fewer coats and faster drying of the polyurethane varnishes. I don’t use them because I don’t like their plastic appearance. They may be faster drying, but there are the other problems as described above. The manufacturers have added a variety of ingredients in an attempt to solve these problems, but they have only succeeded in reducing them while increasing their cost.
Gel Finishes
These finishes have been promoted as “the answer” to our wood finishing problems. Gel finishes are the polyurethane varnish resins without the oil carrier and the solvents. These are the solids that will settle to the bottom of a can of poly-varnish. These finishes will give a more uniform surface coloring because they do not penetrate as deeply into the wood as the liquid varnishes. Other than that, they have all of the same characteristics associated with any other polyurethane varnish. Bartley’s and General Finishes are the most recognized brand names.
