Oil & Varnish: A Refresher

varnish

On the left is a dried puddle of wiping varnish. On the right is a dried puddle of a mixture of oil and varnish. Clearly a huge practical difference between the two.

If you go back to the early issues of U.S. woodworking magazines in the 1970s and ’80s and read about oil finishes, you’ll come away confused. The more you read the more confused you’ll become. I know this because I was there. The claims and explanations were all over the map. More often than not they were contradicting each other.

The problem of course began with the manufacturers and their labeling. What was Danish Oil? What was Seal-a-Cell? What was Waterlox? What was Salad Bowl Finish? For that matter, what was “tung oil,” when brands clearly performed differently? All of these were (are) wipe-on/wipe-off finishes, all are thinned and cleaned up with mineral spirits (paint thinner), and all are very popular with amateur woodworkers. But what are they? How do they differ?

In 1990, at the beginning of my writing career, Jeff Greef asked me to write an article on tung oil for Woodwork magazine (its archives are now owned by Popular Woodworking). I worked for three months researching that article during which the topic morphed into the broader subject of “oil” finishes in general.

I bought all the brands and followed all the directions, applying these finishes to various woods. Still confusing. The breakthrough came when a chemist friend suggested that I put puddles of the finishes on glass and see how they cure.

Voila! Some cured very slowly to a soft and wrinkled film. Others cured fairly quickly to a hard, smooth film.

vboiled linseed oil

As a reference, here is the way a puddle of boiled linseed oil cures on a non-porous piece of glass. Soft and wrinkled.

So there was a huge difference in these products. Some had to have the excess wiped off after every coat or the result would be a gummy, sticky mess. Others could be left damp or wet and built up to a hard film with several coats; this would provide much better water resistance. Some were obviously oil, or they were a mixture of oil and varnish because including oil with the varnish causes the finish to cure soft and gummy. Others were simply varnish, thinned with mineral spirits so the finish could be wiped rather than brushed.

Here’s where the confusion starts because varnish is made with oil. How do you define the difference between a mixture of oil and varnish (Watco Danish Oil, for example) and thinned varnish (Waterlox, Seal-a-Cell or Salad Bowl Finish), which is made with oil, other than by how they perform?

varnish linseed oil

You can’t just mix a natural resin, in this case amber, with oil and expect to get varnish. The resin will just sink to the bottom and remain there. You have to cook the mixture to get the two to incorporate and make varnish.

The key words are “cook” and “mixture.” Varnish is oil and resin (think amber, used as jewelry) cooked to cause the two to combine chemically and make varnish. You can’t simply mix the two; the resin will just sink to the bottom and remain there. You have to cook it. (Varnishes are now made by combining chemicals and heating instead of combining natural products and heating, but the process is easiest to picture with the natural products.)

Once you have the varnish you can mix it with oil (linseed or tung) to make an oil/varnish blend. This blend acts like oil in that it never cures hard. The argument for mixing these, which you can do yourself if you want, is that the addition of varnish makes the finish a little harder and more water resistant than boiled linseed oil or tung oil alone. But my tests indicate that the difference isn’t much.

Never mind, there are still two very different products on the market represented as oil or unclear what they are.

Why did writing about this subject occur to me now, when I’ve written about it so many times before? Because I just received a woodworking magazine with an article that didn’t distinguish this difference. Varnish was defined as a “mixture” of oil and resin.

One of the things I’ve been most gratified about since 1990 is that woodworking magazines have all bought into my explanations and even adopted my term “wiping varnish” for thinned varnish to distinguish it from oil and oil/varnish blend.

tung oil

On the left is Old Master’s Tung Oil. On the right is McCloskey Tung Oil. Two well known companies selling two obviously different products, calling both tung oil. What could illustrate better why finishing can be so confusing?

OK, manufacturers haven’t budged a bit. They still mislead as they always have. But I have come to the conclusion that they don’t actually realize they’re doing this. They are marketing companies. As long as their products are moving off the shelves, everything is OK with the world.

The result of their not caring, however, is that the burden is on us to figure out what it is we’re buying. That’s what made it so depressing for me when I realized that here was a magazine that was still living in the 1970s and 80s.

— Bob Flexner
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9 thoughts on “Oil & Varnish: A Refresher

  1. valentinasmj

    Once an oil is converted into a varnish, do the inherent qualities of the oil remain? For example, raw tung oil is more water resistant, and cures more quickly, than raw linseed oil or soya. But once the tung oil is cooked with resins and dryers, is the resulting varnish any more water resistant than a soya based varnish cooked with the same resins and the same ratio of resins to oil? I ask because I am evaluating a few different varnishes for a kitchen table top that my kids will use daily. Waterlox (tung/phenolic), sutherland wells (tung/uralkyd), and Arm R Seal (which I think is soya/uralkyd). Should I care much what oil was used, or choose on the basis of color, cost, resin, and ease of application? Anectdotally, it seems that the tung based varnishes have shorter open time than the soya (which makes the soya easier to apply but harder to control dust), but maybe it is due to different dryers or resins.

    Also, if you know of any other short oil tung/phenolic varnishes, please let me know. Waterlox is nice stuff by pricey for something that is 50% mineral spirits. It would be cheaper using something like epifanes and diluting myself, but I think that is a long oil and won’t cure as hard.

    1. Bob FlexnerBob Flexner Post author

      I think you’re getting much too involved in the differences in varnishes for what you’re trying to achieve, but you are correct about many of your observations. The solution for your kitchen table, though, is quite easy. Use any oil-based polyurethane from the home center or paint store. It will be more scratch resistant than other varnishes, which you will want for your kids. And apply at least three brushed or sprayed coats. Thickness is everything when it comes to water resistance. For example, three or four coats of brushed or sprayed shellac will be much more water resistant than several coats of any of the thinned varnishes you’re mentioning because each coat is so thin.

      You’re correct that tung oil is more water resistant than linseed oil, but, again, that’s relative because thickness is more important, and you can’t build either because they don’t dry hard. Tung oil was introduced to the West from China a century ago as a better oil for boat varnishes, and many of these varnishes were made with tung oil. But few are today.

      I don’t know of any short-oil varnishes still being made, at least by any definition that used to be used. Back when there was a much bigger market for varnish, a short-short oil varnish (lower percentage of oil relative to resin) had a market for better rubbing qualities because it dried harder. Modern day polyurethane doesn’t rub out as well, but it is tougher and more scratch resistant. (An interesting diversion into the difference between hard and tough.)

      Waterlox is a soft curing varnish, so it is much more easily scratched than any other varnish I’ve used. Arm-R-Seal is hard enough, but you’ll have to apply a lot of coats to get a good thickness. But it’s OK to use.

      I guess what’s happening here is that you have gotten yourself into the finishing world that Fine Woodworking created in the 70s and 80s, “promoting” certain “guru” finishes with articles about them and creating a market that woodworking stores and catalogs have latched onto. You’re thinking that Fine Woodworking knew something special when they didn’t.

      If you were just a typical homeowner who wasn’t acquainted with any of the products you have mentioned, you’d just go to the home center and buy a can of polyurethane with the sheen you wanted and be happy that you have done the best you could.

      If you’re interested in getting deep into oils and varnishes and have a technical library in your area, check out some old books and look through them. It’s a fascinating subject. But you’ll have a hard time making a connection between what you’re reading and what you have access to today.

      1. MikeV

        Thanks Bob – one follow up question if you don’t mind. Does a wash coat of shellac under oil based varnish weaken the varnish is any significant way? The reason I ask is my table will be sapele, which has really open pores. On all my sample boards, I am getting a lot of bleeding. It takes 2 or 3 wiped coats to seal the pores enough to stop the puddles from forming. Yes, I know I could baby sit and wipe away the oil as it weeps out, but I need to sleep at some point! I did a test board with a shellac wash coat, and a single coat of shellac solved most of the problem.

        1. Bob FlexnerBob Flexner Post author

          You won’t cause any problems applying a shellac washcoat under the varnish as long as you keep it fairly thin. The only issue is bonding, the reason to keep the shellac coat thin.

          Actually, I’m surprised you aren’t having drying problems applying the varnish directly to the wood because sapele is an oily wood, I believe. This is another reason to apply the shellac washcoat – to block the oil so the varnish dries quicker.

  2. terrynjon

    This article was timely for me, as I was deciding what to finish a picture frame I needed to deliver the next day when I read it. Thank you for all the information you provide, both here and in your books.

    Jon

  3. Paintress

    First I’d like to acknowledge this as being the best educational “finish” article I’ve read in Popular Woodworking. I have been selling a European line of finishing products which contain zero oils. In fact they believe they not only cause degradation to wood but also pose health hazards. Additionally oils cannot give a clean uniform finish.
    You have pointed out an extreamly common marketing problem what is labeled on the outside of the can is not actually what really inside the can.Try convincing the person holding the can & reading the msds sheet the reality the glass proved. As KeithM said we should be like the textile industry & post detailed information in reguards to what truly is in our products.
    I am glad to see this article. We are focused on our tools & equipment & many other new & improved systems of our trade yet we are still in the dark ages of finishing systems.

    1. Bob FlexnerBob Flexner Post author

      I have no idea what is meant by oils causing degradation to wood or posing health hazards. If the products you’re selling weren’t made with oil, they can’t, by definition, be varnish (see the above explanation). They are something else. If from Europe, they’re probably water-based finishes…using a marketing gimmick that oils are bad for wood and for us.

  4. keithm

    Too bad we don’t have requirements similar to textile fiber identification laws. You have to list composition, by percentage, using generic names, and can’t use terms like “wool-like nylon.”

    One place I usually start is the SDS (MSDS) but sometimes they’re not helpful when they list “proprietary” ingredients. Sometimes I have to cross reference with CAS (Chemical Abstract Service) numbers to realize, “oh, that’s naphtha”

    One of my recent favorites is “synthetic shellac!”

    1. Bob FlexnerBob Flexner Post author

      The SDS (MSDS) is only helpful for the listing of solvents. Finishes such as oil or varnish are rarely listed. So there wouldn’t be any way to determine whether the product is an oil/varnish blend or wiping varnish because they both thin with petroleum distillates (mineral spirits).

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