How to Choose a Finish: Part IPosted on January 27th, 2010 No comments
The first step in finishing a project (beyond preparing the wood, of course) is to choose the finish you want to use. In fact, it’s wise to make this choice even before starting on the project because it may influence the wood you choose.
There are seven broad categories of finishes to choose from:
- Oil and blends of oil and varnish (often called “Danish” oil)
- Varnish, which includes polyurethane varnish, spar varnish, wiping varnish (varnish thinned about half with mineral spirits) and gel varnish
- Shellac, which can be clear or amber and with its natural wax included or removed
- Lacquer, which includes the most common, nitrocellulose lacquer, and also colorless CAB-acrylic lacquer and slower drying brushing lacquer
- Water-based finish, which is often confusingly labeled lacquer, varnish or polyurethane but is an entirely unique finish
- Two-part finishes, which are very protective and durable and which include catalyzed lacquer, conversion varnish, polyester, epoxy resin, UV-cured finish and two-part water-based finishes
- Wax, which can be used as a finish but is not water resistant and therefore usually limited to objects that arent handled much.
These seven can be reduced to four depending on whether or not you intend to use a spray gun. For example, there’s no point spraying slow drying varnish because you can get the same qualities with a faster drying finish and avoid a lot of dust nibs.
So for spraying, limit your choices to one of the faster-drying categories: shellac, lacquer, water-base or two-part finish.
If you’re going to use a brush or cloth, choose among oil, varnish, shellac and water base. Shellac and water base both dry slowly enough to be brushed as long as you move quickly. Also, “brushing lacquer” can be brushed, of course, if you have some reason to choose this product.
Wax is not included in these lists because it has such limited use as a finish.
Now that the choices have been reduced to four, how do you choose among them? There are six characteristics you need to consider. Usually, one or two will totally outweigh the others, so the choice is rarely complicated.
The six are:
- Appearance, meaning the degree of yellow/orange coloring the finish adds to the wood
- Protection (from moisture getting through to the wood) and durability (resistance of the finish itself to damage)
- Ease of application
- Rubbing qualities (how easy the finish is to rub to an even sheen)
- Reversibility (how easy to repair or strip the finish)
- Safety (for yourself, the end user and the environment)
There are two broad categories of appearance: how thin or thick the finish appears on the wood, and the amount of color the finish adds to the wood.
All finishes can be applied very thin to produce what is commonly called a “natural wood look.” Consider Danish teak furniture, which is often marketed as having an oil finish but is actually finished with a very durable, film-building conversion varnish. To get a natural wood look with any film building finish, simply thin the finish a lot to reduce the film build and apply two coats — the first to seal the wood and sand smooth and the second to provide the sheen (gloss, satin or flat).
Oil and wax can’t be built up on the wood because they never dry hard. All the excess of each has to be wiped off, so these finishes are entirely limited to producing a natural wood look. All other finishes can be built up thick.
Different finishes add different degrees of yellow/orange coloring to wood. Most add some amount of yellow/orange, but water-based finishes and CAB-acrylic lacquer add none. Amber (orange) shellac adds the most color. Oil and varnish are next.
Generally speaking, colorless finishes look great on white woods such as maple and ash, and yellowing finishes are better for bringing out the richness of darker woods such as cherry and walnut.
The differences in the amount of color a finish adds are less apparent when used over a stain.
Protection and Durability
Protection refers to water resistanc — ethe degree to which a finish resists penetration of moisture. Durability refers to the strength of the finish film itself to resist being damaged from coarse objects (scratches), heat, solvents, acids and alkalis.
Though the two terms refer to different qualities, the words are often used interchangeably because finishes tend to rate similar for both. For example, polyurethane varnish is both more protective and more durable than nitrocellulose lacquer.
Protection has most to do, however, with the thickness of the film. Moisture has greater difficulty penetrating a thicker film than a thinner film. So four coats of shellac, for example, are far more protective than one or two coats of wipe-on polyurethane (polyurethane varnish thinned half with mineral spirits). But polyurethane is still the more protective and durable finish at the same thickness.
Because oil, oil/varnish blend and wax can’t be built up, they are too thin to offer more than seconds of water resistance.
To better understand protection and durability, please refer to “How to Choose a Finish: Part II.”
Ease of Application
The degree of difficulty applying a finish is a big consideration, especially for beginners and woodworkers who don’t do much finishing.
Speaking generally, wipe-on/wipe-off finishes such as oil, oil/varnish blend, wiping varnish, gel varnish and wax are the easiest to apply successfully. If protection and durability are big considerations for you, along with ease of application, use wiping varnish and apply many coats.
Any of the finishes that dry slowly enough to be brushed are next in difficulty, with the trade-offs being that varnish has more time to level for reduced brush marking but also more time to collect dust nibs. Shellac and water-based finish (and brushing lacquer) provide less time for brushing and also collect fewer dust nibs.
All the faster drying spray finishes are more difficult to apply because of the more complicated care and adjustment of the spray gun. If you have never used a spray gun and do very little finishing, you would probably be wise to stick with slower drying wipe-on/wipe-off and brushing finishes. You can get all the same qualities of appearance, protection and durability with these finishes as you get with the spraying finishes.
On the other hand, spraying is not at all difficult, and if you’re willing to devote the time to learn a spray gun, similar to the time it takes to learn woodworking tools, spraying is fast and produces almost perfectly level and dust-free results as long as you’re using a fast drying finish.
Sanding a finish level and rubbing it to an even sheen is the only way to produce a perfect finish (other than French polishing using shellac). With all the finishes other than oil and wax, rubbing qualities are the opposite of the finish’s scratch resistance because rubbing is scratching using abrasives.
By far, the best rubbing results can be achieved with nitrocellulose lacquer. Shellac can also be rubbed to produce good results. Most other finishes are difficult to rub to an even sheen.
To better understand the reasons for this, please refer to “How to Choose a Finish: Part II.”
Reversibility refers to the ease with which a finish can be repaired or stripped. Easy reversibility is the opposite of good heat and solvent resistance.
The reversibility of a finish is a big consideration for furniture manufacturers and refinishers working on elaborate Victorian and earlier furniture. Reversibility is rarely a an important consideration for woodworkers building new furniture or cabinets.
Furniture factories are very concerned that the finish on the furniture they ship out to furniture stores and eventually to customers can be repaired so the repair doesn’t show. If this cannot be done, the furniture may not be sold or may have to be returned to the factory for refinishing, which is very expensive.
Lacquer and shellac are the easiest finishes to repair invisibly (though the actual process requires a great deal of skill). The repair is usually done by melting solid finish into the damaged area and leveling it out. More finish can also be dissolved into the damaged area. If the finish is resistant to heat and solvents, disguising the repair can be very difficult.
Refinishers are concerned with reversibility because they don’t want some future refinisher to have to sand or scrape to remove the finish they apply. This will damage the wood and the patina (evidence of age), and reduce the appeal, and sometimes the value, of the furniture.
Shellac and lacquer are the most easily reversible of all the finishes. To better understand why, see “How to Choose a Finish: Part II.”
There are three categories of safety when referring to finishes: safety to you during application, safety to the person or persons using what you have finished, and safety for the environment. None of the categories discussed in this article generate anywhere near the emotion and hyperbole as does safety. If you want to get into an argument about finishes, bring up the issue of safety.
For what its worth, here’s what I think.
All finishes can cause health problems during application. The one exception might be solid wax sticks sometimes used by woodturners to finish bowls spinning on a lathe.
It’s always wise to work in a well ventilated room, meaning a room with some air flow to remove solvent vapor. Creating an exhaust is especially necessary when spraying a finish, even a water-based finish, which still contains up to 20% solvent.
But beyond ventilating, unless you are allergic to solvents or do a lot of finishing, it’s very doubtful there’s anything to be concerned about. It’s the chronic (continuous) exposure that is the problem.
Safety for the end user is a non-issue. All finishes are totally safe to eat off of, or even to ingest in small amounts, once they have totally cured (rule of thumb is 30 days, though it is much less in a warm room). The inclusion of lead as a drier in some finishes is sometimes cited as the reason these finishes are unsafe. But lead was removed from all coatings by law in 1978 — though it had never been a problem in clear finishes, only in paint pigments.
Safety for the environment refers to reducing solvents in the atmosphere. It is a big issue for furniture factories and other large users, but I don’t consider it an issue for amateurs or small shops. There’s just not enough solvent being used to be significant. The other considerations: appearance, protection and durability, ease of application, rubbing qualities, and reversibility are far more important.