All About Oil Based Varnish
For many amateur woodworkers or hobbyists who either cannot afford spray equipment or do not have enough space in their shop to set up a spray booth to safely spray finishes like lacquers, brushing on an oil based varnish is one of the best choices for a topcoat finish. Oil based varnish does not dry very quickly, therefore it will take longer to complete a finishing project using varnish as opposed to lacquer, water based or other faster drying finishes. However, if you are an amateur or home woodworker, not a pro who has to worry about meeting a deadline, time should not be top priority, quality should. Varnish has very good resistance against abrasion, wear, heat, solvents and water vapor. Other than its slow drying time, which can also cause problems with dust getting trapped in the finish before it dries, the only other disadvantage to oil based varnish is that it tends to yellow over time. The dust problem can be taken care of by setting up a dust free finishing area in your shop and sealing it off with heavy plastic to reduce the amount of dust and sawdust in that area. The problem of yellowing cannot be handled as easily. First, most oil based varnishes are amber (yellowish) in color to begin with. This is because the oils used to make the varnish are amber. Therefore, oil based varnishes tend to somewhat change the color of the raw or stained wood when they are applied. It is not a considerable change, and unless applied over a very light colored or white stain, it is satisfactory. In fact, amber varnishes actually give darker woods like walnut and mahogany a warmer appearance. Non-amber or what are called water white finishes like some lacquers and water based finishes can leave a cold look on darker colored woods. However, if needed, there are a few water white varnishes available. One is called water white restoration varnish manufactured by H. Behlen & Bro. Nothing can really be done about yellowing of varnish over time and if you think about it, all film finishes break down in one way or another over time and have to be removed and replaced with a new finish. Tung oil yellows less over time than other oils, therefore a varnish that contains tung oil will have a tendency to yellow less over time.
Oil based varnish is manufactured by cooking certain oils that can cure with resins. Once this blend of cooked oil and resin is complete, solvents are added to make it thin enough to apply and metallic dryers are added to help speed up the curing time. Initially, linseed and tung oil were used by manufactures for the curing oils and natural resins like pine and gum resins were used along with solvents like gum turpentine and mineral spirits (to thin it out) and lead used for the drier. These ingredients were not only used to make varnish, but also paint. With the exception of lead, you can sometimes still find some of these ingredients in today's oil based varnishes, but modern varnishes usually use synthetic resins which are superior in strength and longevity and curing oils that are less expensive to use in the manufacturing process along with a blend of solvents and metallic dryers like cobalt and zinc that do not cause health problems such as lead does.
Types Of Oil Based Varnishes
This varnish is made with phenol (a plastic) and formaldehyde. The phenol is a solid and is made into liquid by heating it with oil and then adding in the other ingredients. When the finish is applied in a thin film and exposed to to the air, the solvent will evaporate and it will turn back to its solid form.
Less expensive, this is a type of polyester resin that is combined with alcohol and acid. It is also cooked with oil to create a varnish. This is the most commonly used resin in commercial varnishes today.
Yes, that's right. Polyurethane is classified as an oil based varnish, although some purists will disagree. Initially developed to be used as a substitute for other plastics, polyurethane has become on of the most commonly used resins in the manufacturing of many wood finishes. Polyurethane is a very tough, abrasion resistant resin. There are many types and forms of polyurethane, but the kind of polyurethane finish you are used to seeing in paint and woodfinishing supply stores is not pure polyurethane, but rather an alkyd varnish that has been modified by adding some polyurethane into it. That is why polyurethane should be classified as a varnish. Perhaps a better description would be modified varnish, but nevertheless, still varnish. It is applied and it cures in the same manner as other oil based varnishes. Contrary to what many people say about polyurethane, most modern high quality polyurethanes do not dry leaving a plastic appearance. They are available in various sheens from satin to semi-gloss to gloss and can also be rubbed to a beautiful smooth luster. Polyurethane's abrasion resistance makes it on of the most commonly used finishes today.
Satin Vs.Gloss Or Semi-Gloss
Throughout the years, many pieces of furniture have been finished with varnishes and other topcoat finishes. Years ago, and on much custom furniture still today, the varnish was applied and then rubbed down with steel wool or sandpaper to cut down the shine and give it a more pleasing look. Today, woodfinishing manufactures make varnishes in different levels of shine so the furniture does not have to be rubbed down after the finish has been applied thus saving many hours of hand or machine rubbing. These varnishes are sold in different sheens. Some will give the user a high gloss finish, others like a satin will have a slight gloss. All topcoat finishes start out as high gloss and if the manufacture wants to make a satin or semi-gloss finish, they take the gloss finish and add a flattening paste into the finish along with the oils resins and other ingredients we now know are used to make varnishes. This paste is usually some kind of zinc oxide and it settles to the bottom of the can. This is why you do not have to stir a gloss varnish before it is applied, but you must stir satin or semi gloss varnishes to get the paste off the bottom of the can and mix it into the finish. The flattening paste makes the finish a little duller and prevents the light from reflecting off the surface as much as a gloss finish. The flattening paste also makes the finish less transparent, thus creating a cloudy look. If you apply too many coats of satin or semi-gloss varnish, you could actually start to obscure the grain of the wood. Whenever I elect to use a satin or semi-gloss varnish, I will use gloss varnish and then only on the last one or two coats use the satin or semi-gloss, this way I can keep the clarity and still achieve the desired sheen. Satin and semi-gloss varnishes are also softer than gloss varnishes because the flattening paste or agent used will soften the film finish. If you need a really hard abrasion resistant finish, but want a satin or semi-gloss sheen, it's best to use a gloss and after it has cured, rub it down with steel wool or other fine abrasives. This will also give you a smoother surface, removing any dust nibs and leveling the surface.
What Varnish Should I Use For My Job?
To determine what type of varnish you should use for a particular job, you must look at what type of oils and resins are contained within a varnish and what the ratio of oil to resin is. Varnishes that contain a larger amount of oil to resin are called long oil varnishes. Varnishes that contain a lower amount of oil are called medium oil varnishes. Long oil varnishes are more flexible than medium oil, but also softer. Medium oil varnishes are harder, but are more brittle. For exterior use, a long oil varnish is best. Because it is more flexible, the varnish will expand and contract with the wood as changes in temperature and humidity take place. A medium oil varnish will not move as much and therefore as the wood moves and the varnish does not, the varnish will soon start to crack and peel. Medium oil varnishes are best used indoors where a lot of wood movement does not occur and a harder finish is desired. The resin contained in a varnish is also important in determining what type to use for your project. Some resins are more elastic than others, making them best suited for exterior uses. Phenolic resin is more elastic than other resins, therefore it will be able to withstand the extreme wood movement of exterior projects without quickly breaking down and cracking. Alkyd and polyurethanes are better suited for interior use. Not as important, but still a factor is what type of oil is used. Tung oil is more water resistant than linseed or other oils, therefore it would be a better oil for exterior use, but much more expensive.
Putting this all together, we basically come down to two categories, interior and exterior use. For exterior use, a modern spar varnish which is long oil and is made up of tung oil, Phenolic resins, solvents, dryers an Ultra Violet blockers (to protect the color of the wood from fading) is probably your best choice if you elect to use an oil varnish outside. Although Spar varnishes have a tendency of initially being more amber (yellow) in color because of the color of the Phenolic resin. For interior use, my favorite is polyurethane modified varnish. The best I used is a product called Wood Glo, it is a satin poly that flows out beautifully and lasts decades. It is sold by Constantine's in New York (See Sources) back on my homepage. They also have a gloss version called Super Shield.
If you have any other questions about varnish, please feel free to e-mail me