Wood Finishes

There are as many different ways to classify wood finishes as there are people to make up the classifications. For furniture, let's break finishes down into two classes, with subgroups, the two classes being clear and opaque. Clear finishes would include lacquer, shellac, varnish, tung and Danish oils and linseed oil, as well as polyurethane. Opaque finishes would include paint (both oil and latex), as well as some lacquers. Neither of these lists is all inclusive, but it covers the range of what you'll commonly find on cabinetry and woodwork.
Another way to classify finishes is by the way they "set up." Lacquer and shellac (retail) set up purely by drying; they do not change chemically. The solvent for either one will dissolve the finish. I sometimes use lacquer thinner as a stripper on pieces finished in lacquer; it's easier, less hazardous, and more economical. Both of these finishes are also anhydrous, which simply means they will absorb water. The resulting white water marks can usually  be removed fairly easily. Other finishes change chemically when they dry. Paint, when dry, cannot be restored to a useable liquid; neither can polyurethane or varnish.
The lacquer commonly used in commercial finishes these days is more often than not catalyzed lacquer. What this means is a catalytic agent is added to the lacquer to make it dry into a more durable finish. Catalytic lacquers come in two varieties, pre and post. Pre catalyzed lacquers have the catalytic agent added to the lacquer at the factory. The catalytic action begins when the material is opened (exposed to air). Post catalyzed lacquers have the catalytic agent shipped in a separate container, for addition to the lacquer at the job site. The working time (usable life) of either of these is normally no more than 48 hours, sometimes less, depending on the manufacturer. Pre catalyzed lacquers are normally used by large manufacturers who can be assured of using up an open container before it's time is up. Post catalyzed lacquers are used by many refinishing shops because it allows them to mix up what they need at the time without wasting the rest, and at the same time getting a finish superior to ordinary nitrocellulose lacquer.
Your choice of finish when redoing a piece is determined by a number of factors; use, appearance, and value being the foremost considerations. You wouldn't want to use shellac on a dining room table top - it's too fragile to hold up. If you've got a piece with pretty grain and a nice natural wood color, you probably wouldn't want to paint it. In short, there are hundreds of variations you can use when finishing a piece of furniture. Consider what's important to you - durability, beauty, ease of maintenance, etc., in selecting the finish you use. Here then are the more common finishes available to the home owner, with what I perceive as their benefits and faults.

Lacquer - Clear finish best suited for showing off wood grain.

  • Positives: Available in a variety of sheens, from flat to high gloss. Easily applied with brush or aerosol. Dries quickly (with a brush, you have to work fast.) Most retail brands require no substrate sealer. Damaged finishes can usually be repaired without stripping.
  • Negatives: Easily scratched and susceptible to water damage. Lacquer is the finish used on 99% of all commercially manufactured furniture with a clear finish.

 

Varnish - A clear finish.

  • Positives: Much more durable than lacquer. Slow drying (allows more time to work). Most minor damage can be repaired without stripping.
  • Negatives: Slow drying time allows dust motes to settle in finish. Tendency for beginners to 'over-brush' when applying the finish, resulting in brush marks in the dried finish. Although you can handle a varnished piece the next day, varnish hasn't cured completely until about a month later.

 

Polyurethane - A clear finish.

  • Positives: More durable than either varnish or lacquer, and easier to apply than varnish.
  • Negatives: Improperly applied finish usually must be stripped, unlike lacquer or varnish which can many times be "worked on" without stripping. Extremely difficult to repair scratches and chips - repair is not for the amateur. Sometimes difficult to strip.

 

Shellac - A clear finish rarely used as such today except in restoring period furniture.

  • Positives: Brilliant shine.
  • Negatives: Highly susceptible to damage from almost any liquid, including alcohol (mixed drinks will cut right through it), fruit juices (ditto), even water will damage it if left to stand. Shellac is used primarily today as a sealer and under coat. It can be used under lacquer or varnish, as well as some polyurethanes.

 

Latex Paint -

  • Positives: Easy to apply, easy to clean up. Suggested for any painted furniture where extreme wear or abuse is not a factor.
  • Negatives: Sometimes difficult to clean a piece entirely when stripping. Repairing chips and scratches on older pieces may present a color match problem. On raw wood a primer is necessary.

 

Oil Based Paint -

  • Positives: Extremely durable. Suggested for children's furniture and any other application where severe abuse may be expected.
  • Negatives: Same as latex paint with the addition of a somewhat messier cleanup.

 

Tung/Danish Oil -

  • Positives: Inexpensive, easy to apply, durable, water-resistant.
  • Negatives: A smooth finish takes a good number of coats. Slow drying.

 

There are other choices in addition to these, of course. Remember your main considerations: use, durability, aesthetic appeal, ease of application, and you'll pick the right finish.

 

Application of Wood Finishes

BLUE MOUNTAIN CUSTOM WOOD PRODUCTS uses a variety of application methods for wood finishes.  The methodology I choose for a particular project is generally determined by the type of finish desired, the specie(s) of wood used and the depth of profile detail.

Often, a particular project will require several application techniques, depending on the various stages of the finishing process.  For  most (but not all) final topcoats I will use an HVLP system of the highest quality available to ensure a Grade A finish.

Because many people have asked me to explain what an HVLP system is, I've included a detailed explanation of the various types below.  The type I've chosen is an internally atomized, dual-regulated pressure tank system.  Atomization and fluid flow is further regulated at the gun, and the typical fluid aperture I use is a .1mm.

 

High-Volume Low-Pressure (HVLP) Atomization

Worldwide concern over increased air pollution has necessitated numerous changes in finish spraying technologies and created a sort of "space race" among spray system manufacturers to provide environmentally cleaner, more efficient systems which produce finishes far superior to what was available just a few years ago.   These technologies have proved to be of great benefit to me (and hence, my clientele) in the form of how I finish my products. HVLP air atomization, Air Assisted Airless and electrostatics are now the only accepted methods of production spraying in certain parts of the country. 

Although all HVLP spray guns operate with the same objective in mind, how they accomplish this goal may differ. First, air used in the atomization process reaches the HVLP spray gun's nozzle in one of four ways: (1) standard high-pressure compressed air, which has its pressure restricted within the gun body; (2) standard high-pressure compressed air, which is assisted with a venturi feed and then filtered ambient air prior to its pressure restriction within the gun's body; (3) standard externally fed HVLP turbine air; and (4) compressor-assisted externally fed turbine air. Items 1 and 3 have seen the most growth and ultimate acceptance in recent years.

All HVLP spray guns should operate at air pressures between 0.1 and 10 psi (at the air nozzle) and consume air volumes of 6-30 cfm (cubic feet per minute) to be considered true HVLP spray guns. Although some HVLP guns with internal restrictions can exceed 10 psi air pressure, it is up to the operator to follow local regulations when necessary.

The benefits of HVLP atomization are improved transfer efficiency, often approaching 65%, compliance with local finishing regulations, a softer spray that penetrates easily into recesses or cavities, reduced material (costs) consumption as well as reduced spray booth maintenance and reduced hazardous waste. Turbine-operated HVLP systems enjoy great portability and ease of operation where compressed air is not available. HVLP spray guns with internal restrictors use existing air supplies, are easy to operate, and the newer, dual-regulated pressure tank systems provide a finer finish than a standard turbine HVLP system.