There are as many different ways to classify furniture 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 oil, 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 furniture.
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. These white water marks generally can 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 attributes 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.
- 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.
- 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. Just remember the main considerations: use, durability, esthetic appeal, ease of application, and you'll pick the right finish.