CWB August 1999

Selecting the Right Coating

There are many different coatings on the market, each with different attributes. Finishers should make a careful choice, based on the specific requirements for a particular project.

By Mac Simmons

Selecting the right coating is just as important for the finisher of a project as selecting the right species is for the cabinetmaker who builds it. The cabinetmaker chooses certain woods for their beauty, strength or structure, depending on the type of work he is doing. The finisher has available a variety of coatings from which to select and he also makes a choice based on the types of woods being used and the finishing work being done.

Finishers should be aware of the various coatings that are available for different types of work and species of wood. Following is a look at some of the most common coatings.

A finisher should begin by making a decision as to the color he wants for his clear coatings. Many clear coatings have an amber cast, and many are "water-clear" coatings. The water-clear coatings are completely transparent - they are clear like water in their appearance both before and after they are applied. Most are non-yellowing and will remain water-clear for years to come.

Amber-cast coatings can be used on most woods and on finishes that have been stained in almost any color, except for the whites and other light colors like blond, champagne, antique and pickled whites. When amber-cast coatings are applied, they begin to add a yellowish color into the finish. This will change the overall color of natural woods as well as some light-colored stains, which will affect the final finish.

In addition, the amber cast in these coatings will usually darken as they age. In some cases on colored stains or on some of the natural woods, this amber color can add character to the woods and enhance the appearance of their finishes.

Water-clear coatings can be used on every color stain, including all the lighter-colored stains, all shades of white and lighter-colored opaque lacquer coatings (base coats). In fact, all of these lighter-colored stains should be clear topcoated with water-clear coatings.

The opaque-colored lacquer coatings are usually referred to as a base coat/clear coat finish. This means that first, a colored base coat is applied, followed by the water-clear topcoats. This gives added protection and adds to the depth of the finish.

Whenever you select a clear coating on natural, lighter-colored woods with a natural finish, it's a good idea to make up

This is a natural panel shown, to which an amber-cast coating has been applied. The woodgrain has been enhanced by the amber coloring.

samples using both an amber and a water-clear coating to see which one looks best. As all amber-cast coatings leave a yellowish cast of coloring on the lighter woods, the more coats you apply, the more amber the color you will see on the natural woods. (See photo.)

Some of the coatings that are water-clear in color include: most water-reducible coatings, conversion varnish, acrylics and butyrates, and hybrid C.A.B. coatings, which are a combination of cellulose/acetate/butyrate resins. While this is a good general guideline, always check with your supplier to be sure that the coatings you select are true water-clear coatings.

All of these are single-component coatings except for the conversion varnish, which is a two-component catalyzed coating. It is very durable and has excellent chemical resistance. (Two-component coatings contain two parts - a resin and a catalyst - that are mixed together. This creates a chemical reaction in the coating that crosslinks the chemical molecules to make these finishes more durable and chemical resistant.)

Amber coatings include the "old standard" nitrocellulose lacquers, which are fast-drying and alcohol-resistant, plus polyurethanes and varnishes, which are slower-drying and used for durability. Another choice is a hybrid coating that combines nitrocellulose and polyurethane resins. This coating has the speed of drying from the nitrocellulose and the durability and chemical resistance of the polyurethane.

All oil finishes, including modified varnish oils, are amber in color, including tung oil, boiled linseed oil, antique oil and Danish oils. In addition, a pigment or dye colorant is added to some oil finishes, which adds color and finish with each application.

There also are many other coatings that are available, ranging from various two-component catalyzed coatings to one-component precatalyzed coatings (this is a single-component coating that has a catalyst already added during the manufacturing process).

Because of all the ongoing advances in chemistry, some of the traditional amber-colored coatings are now available in

water-clear versions. That is why it is important to check with your finishing materials supplier when making a coatings decision to ensure that you get exactly what you want.

There are other factors in your finishing specifications that must be given consideration when choosing a coating; for example, whether an exterior or an interior coating should be used. High-moisture areas, such as kitchens, bathrooms and basements, require good moisture-resistant sealers and coatings.

Other considerations include:

* Is the coating easy to apply?

* Is the coating economical, taking into consideration the work that is being done?

* What is the coating's ease of sanding, rubbing and polishing?

* What is the coating's appearance of sheen? What is its clarity?

* Is the coating compatible with other finishing materials you will be using?

* Can the coating be touched up and repaired if it is damaged?

* What is the dry time?

* What is the durability and the resistance to chemicals and marring?

* Does the coating have the ability for a fast buildup? What is the amount of solids content in the coating (the dry film that remains on the wood after all the solvents and extenders have evaporated)?

* Is the coating ready to spray, and what is the proper reducer/thinner to use and the correct reduction ratio of the coating?

* Is the coating non-yellowing? Does it contain UV inhibitors or blockers?

You may also want to ask if pigmented colorants and dye stains can be added to the coating to make up colored tinting toners and shading stain lacquers. All these variables should be discussed with your coatings supplier.

In some cases, you may have to make some allowances, since all coatings have different characteristics. One single coating may not have all the features you want in a finish. For example, certain coatings may require different types of sealers. All coatings that use a catalyst should use only a single- or two-component vinyl sealer (which are excellent moisture-barrier sealers) or sealers that do not contain any stearates. Also, there are some coatings that work best when you use the same coating as its own sealer, by thinning out the first and second coats to act as a sealer and using the same coating for completing the work. Doing this assures that these coatings will always be compatible with each other.

I also want to mention the so-called "V.O.C." coatings, also known as compliance coatings. They are all manufactured ready for use, which means that no thinners are allowed to be used to reduce them, as that would change the volatile organic compounds that are emitted into the atmosphere. Adding any reducer, including certain additives, would make these coatings noncompliant.

There are other coatings that must be thinned out to achieve the best application from them, and to get the best flow out and prevent orange peel. Using the proper thinners and other additives will make these types of coatings work as the manufacturers intended.

In many cases, one coating may be used on most of the work done in a shop. But do not expect one single coating to be suitable for every type of work.

Selecting the coating should be an important part of planning each project. It should always be discussed with the finishing department from the beginning when you first make a proposal to do a job.

Once finish samples have been approved and the order has been confirmed, then the finishing materials should be ordered, just as you order your lumber. Your finishing materials should be on hand when you begin to go into production and as soon as the pieces have been completely fabricated, then your finishing process can start. Selecting the right coatings will be right for you and also right for your customers.

 

Have something to say? Share your thoughts with us in the comments below.