Practice truly does make perfect with the right finish

I’m often asked, “What’s the best finish to use?” It would be great if I could answer that question simply. But there are so many variables to consider, many of which can only be answered by the end user or someone who is performing the finishing for the end user.

For the end user, the first question is “What do you want it to do?” followed by, “What do you want it to look like?” In some cases, the questions can be reversed in priority. After that we look at all of the factors leading to a good decision.

We finish a wooden object to enhance or modify its looks, to bring out characteristics that we want to emphasize or to play down less desirable characteristics, to preserve and protect it from environmental factors and to increase its utility by making it more durable.

Often a finish that is very hard and mar-resistant will crack or craze when applied over wood that moves more than the finish can accept. A super finish in terms of performance might use a hazardous chemical catalyst requiring expensive application and safety equipment, making it unsuitable for your setup. An easy-to-apply finish, such as many wipe-on oils and varnishes, might not have good moisture resistance or not be able to provide the rich look of a multistep lacquer finish. A lacquer finish might be beyond the skill of some, but will be preferred by the finisher with experience and the equipment to do a good and profitable job.

There is no one finish that meets all the possible needs for a wood finish, although most finishes will satisfy some or all of the requirements to an extent. In this respect, we can consider them all somewhat of a compromise. So the end user needs to determine his needs and priorities. How will the object be used? A dining table in a household with children might be best served by a tough, moisture-resistant and low-maintenance coating. A sideboard in a formal dining room seldom used by an older couple would look great with a rich, deep, well-rubbed-out nitrocellulose finish. A piece in a bathroom will need good moisture exclusion properties as well as a liquid-resistant finish. A blonde, whitewashed or pickled finish will need a water-clear and non-yellowing finish. Chairs in a sunroom or enclosed porch would benefit from a finish containing ultraviolet inhibitors.

Film study

I’m going to exclude oils and waxes from this discussion and concentrate on film finishes (those that form a film on the surface). The two main types of film finish are evaporative coatings (those that dry via evaporation of the solvent) and reactive coatings (those that dry and cure via a chemical reaction between the resin and either a chemical catalyst or oxygen). Evaporative coatings can be dissolved in solvents similar to those used in the liquid coating and are softened by heat. Reactive coatings are more tolerant of heat and cannot be readily dissolved with solvents once they have cured.

Evaporative coatings such as shellac, nitrocellulose lacquer, acrylic lacquer, cellulose acetate, butyrate lacquer and many hybrids are more forgiving in application, generally more versatile in terms of the variety of looks they can provide, are easier to sand and rub out and, because they can be dissolved by solvents, are easier to repair. While modern nitrocellulose lacquers are quite suitable for indoor furniture, they are easier to scuff or scratch and less moisture- or chemical-resistant than reactive finishes. Shellac is especially susceptible to alcohol and water.

Reactive coatings include oil varnishes (which cure via a reaction of the oils and resins with oxygen), post- and pre-catalyzed lacquers, post-catalyzed conversion varnish, catalyzed urethanes, polyester and epoxies. In general, the reactive coatings are more hazardous, require more careful handling and personal protection and have a shorter pot- and shelf life. They are more difficult to repair and many of the sophisticated coloring techniques used with straight lacquer are more difficult to perform well.

Most catalyzed coatings must be applied as self-sealing or over a non-stearated sealer such as vinyl. Acid catalysts require glass or stainless-steel components for contact with the liquid or the catalyzed coating and can cause a reaction with some dyes.

Catalyzed coatings, especially post-catalyzed versions, because the cured film is not dissolved by the original solvents, must be either recoated before the cure has proceeded too far or, failing that, allowed to cure for a much longer period of time before sanding to establish tooth. This is called a recoat window and will be shown on the product label or product data sheet as the interval between the recommended dry time at a specific temperature and the maximum cure time before recoating can be done. If the recoat window has been exceeded, you will no longer achieve a solvent bond between the new coat and the coat below. You must abrade the subcoat in order to achieve an acceptable mechanical bond. There might be limits to the number of coats to be applied at a given film thickness, within a certain period of time, or a maximum dry film thickness to be achieved.

Product properties

Here are some general properties of various coatings available for wood:

Epoxy: Very tough, heat- and chemical-resistant and tricky to apply. Used for some marine applications and as a very thickly poured clear coating on tables and bars.

Polyester: Very tough and hard two-component coating. Can be applied very thick. Proper application procedures must be followed.

UV coatings: Use ultraviolet light to start a chemical reaction that can cure the film in seconds to minutes, but requires rather expensive equipment. Very high solids formulations mean fewer coats and less solvent to achieve desired film build.

Polyurethane: Two-part urethanes are increasing in popularity for very durable finishes and are becoming somewhat easier to use. Pot life might be much shorter than with conversion varnish. Respirators supplied with outside air and complete skin protection for the applicator is usually required.

Conversion varnish: On cabinetry, it is usually the next step above pre-catalyzed lacquer in durability and popularity. Longer pot life than polyurethane. More critical application constraints than pre-catalyzed lacquer in number of coats, catalyzation ratio, temperatures and recoat intervals.

Post-catalyzed lacquer: A step down from conversion varnish in film performance, but more versatile and less expensive.

Pre-catalyzed lacquer: Probably the largest and fastest growing segment of finishes for furniture, fixtures and cabinetry. As easy to use as a straight nitrocellulose lacquer but more durable and moisture and chemical resistant.

Waterborne lacquers: They are generally clear, relatively non-yellowing, and can have decent rubbing properties. Some grain raising can be expected, while dry times are extremely responsive to temperature, humidity and air movement.

Butyrate lacquers: Resistant to ultraviolet light and yellowing. Often combined with acrylics for a water clear coating.

Nitrocellulose lacquer: The workhorse of the furniture industry for many years. Good appearance, easy to sand, polish and repair.

Shellac: Alcohol soluble resin. Low odor, fast drying, easy to apply and repair, but not as durable as lacquers. Used primarily in restoration.

Go to the source

Most coating manufacturers have product data sheets available online. While not all are as comprehensive and detailed as we might want, some are very well done and provide objective data concerning performance we’re interested in, such as hardness, flexibility, sheen, solids content, resistance to heat, solvents, acids, alkalis, salts and water. Some of the best contain results of standard tests performed to give you metrics for comparison with other coatings.

Much of what you might find in online discussion groups will be opinion, guesswork or outdated information. The coating manufacturer can be held accountable for inaccurate data or data that can’t be substantiated, so I’d give those more credence than a simple Google search.

Finally, read the product’s directions and follow recommendations. Create a step panel using the whole finishing schedule to make sure you can achieve the desired finish. Don’t expect to use something for the first time and get it perfect. 

Greg Williams, formerly senior touchup and finishing instructor for Mohawk Finishing Products, is now a freelance instructor and consultant. He can be reached at gregalwil@yahoo.com.

This article originally appeared in the June 2014 issue.