General Varnish Question

[Fred Seider]

I'm a little confused on the term 'varnish'. I'd like to know what is and what is not a varnish. I know what an oil is - BLO, tung, etc. And that shellac (Bullseye) is not a varnish, but what about a wipe on poly (minwax), or other products?

Thanks,
Fred

[Sam Yerardi]

Traditional varnishes are comprised of a drying oil (linseed, tung, walnut, etc.), a solvent (typically turpentine), and a resin (copal, rosin, dammar, amber, etc.) of some sort. They dry and cure by evaporation of the solvent and harden thereafter.

Shellac is actually a varnish and is chemically known as a single component resin varnish, where the 'resin' is what is secreted from the lac bug.

Newer varnishes typically contain an alkyd, which is a modified vegetable oil or other higher-level oils, some UV-blocker additives, and phenolic resins.

Polyurethane varnishes are chemically different than traditional varnishes and cure differently. They react chemically after evaporation of the solvent or by reaction with the air. There are many different formulas for polyurethane varnishes.

Acrylic varnishes are usually water-born varnishes.

[Howard Acheson]

There are a number of oil based varnishes. They are all made basically the same way. A resin and a drying or semi-drying oil are mixed and then heated to a certain temperature at which point they combine into a new compound called "varnish". This varnish is very thick like a syrup so a thinner is added to bring the varnish to a brushing or wiping consistency. The resins used are alkyd, phenolic and/or urethane--all are plastics. The oils used are linseed, tung or soya. The thinning chemical is mostly mineral spirits with some other additives like driers to help the varnish cure faster.

Each of these components bring some unique characteristics to the party. Also, the proportion of oil to resin affects the hardness and flexibility of the varnish. The more oil, the more flexible. Marine varnishes are "long oil" to allow them to flex with the varying conditions which they have to contend with. Interior varnishes are "short oil" and are therefor harder and more durable. Polyurethane resin is a popular resin in varnish. Poly resin has become popular primarily because it is cheap. Some poly finishes are 100% poly resin while others are blends of poly and other traditional resins. It is also somewhat more scratch and heat resistant but it has some serious appearance issues. It's fine for floors and surfaces like children or work areas where the finish might get abused. It has a cloudy look and, when the film thickness is excessive, it looks like plastic. Phenolic resin varnishes are as durable and have a better appearance. Alkyd/soya varnishes are almost water clear while any varnish with linseed oil will be quite amber in color.

Finally, oil based finishes all dry and cure the same way. First the thinner evaporates and the surface becomes tack free. Now oxygen begins to react with the varnish causing the varnish to change from a liquid into a solid. That's the reason oil based varnish is called a "reactive finish". This curing process takes from 4-12 hours depending on the temperature and the thickness of the film at which time the curing process is about 80% complete. Full curing takes 3-4 weeks for most varnishes. Being a reactive finish, varnish can not be redissolved by its thinner unlike evaporative finishes like lacquers and shellacs. That's the reason it is so much more durable than evaporative finishes.

Solvent finishes are called "evaporative finishes' where the resin base is dissolved by a solvent. Lacquer and shellac are the two primary evaporative finishes. When the solvent evaporates a thin film of the resin is left on the surface of the item coated. No secondary reaction occurs so the finish is able to be used almost immediately. However, the finish can be dissolved by the solvent so it can be easily damaged.

Finally, the newest type of finish is called a "coalescing finish". These are the acrylic waterborne finishes. The finishes carry names that would have the buyer believe that they are just a water based variant of their oil or chemical brethren. So you see "waterborne varnish", "waterborne shellac" and "waterborne lacquer". In fact, they are all acrylics with little or no actual resin from their namesakes. The most popular is a urethane enhanced acrylic called "poly", "polyurethane" or "polyacrylic". The chemistry of waterborne finishes is quite complex. Water is the carrier of the chemicals (it's not a thinner). There are three steps to the drying/curing process. First the water evaporates, allowing the other chemicals come into contact and sort of melt together (coalesce) flowing out into a film. After coalescing the chemical react or cure into a solid film. Many think the the rapid evaporation of the water indicated that the finish is fully ready for use. However, it takes 3-4 weeks for a waterborne acrylic to develop its full adherence and protective qualities.

Let me suggest that you purchase Bob Flexner's Understanding Finishing to learn about finishes and to learn finishing. It is an excellent book and will get you well down the road.

[Steve Schoene]

As Sam has pointed out there are a wide range of materials that have been given the name varnish. The definition is an evolving one, as most definitions are.

I often find it more useful to tighten the definition to its more modern dimensions. That would eliminate evaporative finishes such as shellac and lacquer, in part because of the burn in between coats that affects how they are applied, and because they always remain soluble in there basic solvents.

That leaves three major resins used for oil-based materials. Phenolic, Alkyd, and Polyurethane. Phenolic resin is the plastic called Bakelite whih is cooked with oil to become a varnish. Aklyd resin, is a polyester formed from reacting an alcohol with an organic acid. In single part air curing finishes polyurethane doesn't appear alone, but is combined with alkyd resins and often called uralkyds.

These three categories of oil based varnish are all quite similar in the way they are applied and how they cure. If you know how to apply one of them you don't need to change anything to apply any of the others. That is why I don't find it useful to distinquish between aklyd and phenolic resin (the traditional resins) varnishes and polyurethane varnish. All are equally varnish.

They do have slightly different properties. Phenolic resin is tough and relatively UV resistant, which is why it is often used in marine spar varnishes. It does yellow significantly over time. Alkyd resin is the most widely used, and yellows less and while not as tough as phenolic resin is still plenty tough. It is relatively cheap to produce which is one reason that it is so widely usedf. The addition of polyurethane adds scratch resistance, but makes a varnish that is more difficult to rub out, is more sensitive to problems that create adhesion problems, and can be a bit cloudy in thick coatings. You should sand between coats of all varnishes, but be particularly careful to do this with polyurethane varnishes.

Wiping varnish isn't anything really special, it is just any varnish thinned to a consistency that can be applied with a cloth. It just takes about three wiped on coats to be equivalent to one coat of "full strength" brushed on varnish.

Waterborne finishes are a horse of a different color since they are emulsions in water that cure in a two step process. First they coalesce as the water that holds the emulsified packets (latexes) of finish apart evaporates away. Then a reactive process of curing begins. I wouldn't be too eager to classify them as varnish or lacquer, as the manufacturers have done. If anything, finishes that burn in between coats even within a "window of time (such as Target USL) I would be fine calling lacquers. If not call them something else.

[Sam Yerardi]

Fred,

Do yourself a major favor and save the answers that Howard and Steve have given you, because I know I will. Extremely good information, and excellent advice on getting Flexner's book. I use it religeously. Also anything written by Jeff Jewitt.