Let's first start with "what" a varnish is. The definition that most consistently applies is that a varnish is a protective coating comprised of one or more resins dissolved into a solution which can easily be applied to a surface. There are two main categories of coatings that all types of coatings fall under, albeit with some caveats and footnotes; evaporative and reactive coatings are these main two categories (in the coatings industry, non-convertible and convertible respectively are often used, but the preceding terms are in more common use and are easier to explain/understand). Evaporative coatings like shellac are deposited via solvent/carrier evaporation. Reactive coatings cure by chemical reaction, oxygenation, moisture, radiation, etc. Evaporative coatings, like shellac, can be reversed at any time by simply using one of the main solvents originally used to keep the coating in a workable, liquid state. Reactive coatings, which include oil-based varnishes, must be removed with strong solvents after they are cured, i.e. with paint remover.
The original varnishes were natural resins dissolved into spirits of wine, i.e. crude ethanol. Shellac, mastic, sandarac, and manila copal are all examples of these alcohol soluble types, but so are rosin and dammar which are soluble in turps. These varnishes acquired the label "spirit" varnishes, but they are all evaporative coatings. Folks were puzzled as to why some very similar looking, hard resins did not dissolve in alcohol or turps. Amber was one frustrating example of this kind of resin. Amber is often quite transparent and rather hard and durable so it seemed like it would make a good varnish, but folks could not get it to dissolve like the other resins. We now know that amber is one of several fossil resins which are found below ground and are tough because of the minerals from the soil that have leached into it over long periods of time. For quite a few years before, literally thousands, folks were also using oils from vegetation which they observed completely dried (eventually) to a film and offered pretty good protection from the elements. It was noticed that when these oils were heated, they dried much better/faster. One day in history, some person decided to add the harder (and somewhat useless before then) resins to the heated oil. Quite possibly attempts were being made to do this with the more familiar resins in use at the time, with mixed results - we'll probably never know for sure, but speculations abound. Anyway, the hot oil allowed some of the resins to be quite readily dissolved, just like one could do with resins like sandarac and "shell-lac" in alcohol. Viola, another type of varnish was born, the reactive type also known as the oleoresinous varnish.
Oil/resin varnishes are reactive because of the oil, which is itself a reactive finish. Drying oils such as Oiticica, Perilla, Tung and Linseed all cure by polymerization. The polymerization can be in the presence of oxygen, but this element is not always required for polymerization of the oil. What is polymerization? Chains of repeating molecular patterns called "mers" in organic chemistry, bound together in one "macro-molecule" are called polymers. It is always by chemical "reaction" that these mers or dimers come together to form polymers, and these polymers link to other polymers, to form the macromolecules. The drying oils, whether partially polymerized to begin with or not, complete the linking process to form a "dry" film, something we have found quite a use for. As it turns out, the resins in the varnish can be part of the polymers too, i.e. chemically bound to the triglycerides that comprise the oils themselves. These are usually referred to as oil-modified resins, but it's almost more convenient to think of these as resin modified oils! It's also true that many resins actually do not become chemically bound to the oil molecules, especially by merely cooking the two components together - they are simply in solution with the oil even as it changes to form a dry film, much the way cement, sand, and crushed stone are combined to form concrete. Oh, did I forget to mention that varnish films are plastic? Yes, even without polyurethane resins, all varnishes share a common ancestry with plastics. Today, synthetic resins are used which are superior in terms of cost, reliable supply, and product performance over the natural resins taken as a whole group.
Now, what's all this have to do with Watco, Minwax, Waterlox, etc? Well, when it comes to reactive oil/resin varnishes, these products are all the same sort of stuff. Another useful categorization of varnishes, just the oil/resin type, is oil length - long oil, medium oil, and short oil . Here's how the three types are defined: short oil - less than 45% oil content medium oil - more than 45%, but less than 60% oil content long oil - more than 60%, but less than 75% oil content, and some describe a varnish with an oil content greater than 75% as a "very long oil" varnish. The world has pretty-much settled on formulating short and long oil varnishes. Short oil varnishes are your typically thick, hard drying varnishes intended for interior use in particular. Long oil varnishes are typically used outdoors and anytime some flexibility is required in the coating. The wiping varnishes are all either "thinned-down" long oil varnishes, or short oil/drying oil combinations. Therefore you can take a product like Minwax Spar urethane, which is a blend of tung-modified urethane resin and phenolic resin, and thin it down to a workable consistency - it becomes a wiping varnish purely as a function of viscosity like all the wiping varnishes. You can take a varnish like Pratt & Lambert # 38, a Soya-modified alkyd (the oily part)/rosin-modified alkyd (the harder part), and combine it with mineral spirits and boiled linseed oil - now you have another wiping varnish recipe. In both cases, the oil, now possessing a low viscosity, will penetrate and beautify the wood. You'll also have pretty-much Watco natural using a recipe like this last one and adding a little kerosene. Add some asphaltum (non-fibered roofing tar) dissolved into mineral spirits or kerosene and you can make any of the "Walnut" tones of Watco depending upon how much tar you add.
So you see, applying pure oils like Tung or Linseed, then top coating with an alkyd varnish for example, is kind of a wasted effort. The oil/oil-modified resin is always there, or can be added to achieve the same result - and BTW, the durability of the resulting film depends solely on the type of oil and resin used. Tung oil is the best for all-around performance, and the phenolic, urethane, and alkyd resins all provide excellent durability characteristics. With a Tung oil/phenolic resin blend, you get a component in the Tung oil that binds very well to the cellulose of the wood and a component in the phenolic resin that binds very well to the lignin of the wood. This is why I particularly favor Waterlox Original (AKA Transparent), which is 86% Tung oil with non-reactive phenolic and natural fossil resins (before thinning) - it creates rather unique optical effect that most other varnishes cannot duplicate and it can be intermixed with a variety of other substances to modify it for a variety of purposes.
©Russ Ramirez 2000 All rights reserved.