If you are like most paint sellers, you probably get a lot of questions about clear coatings... and understandably so. Clears can be confusing, what with the many types of these coatings, and their use in a wide variety of applications, ranging from furniture finishing to fences to fishing boats.

What are the differences between varnishes, polyurethanes, lacquers and shellacs? Under what circumstances should you use a solvent-based clear rather than a water-based coating? What is the proper way to apply clear coatings?

If you hesitate when asked questions about clear coatings, it may be time to bone up on the subject. Here are some basics to get you started.

How Clears Differ From Paints

The very first thing to understand is how clear coatings differ from paints. In a word, paints contain pigment, while clears have none, or almost none. Otherwise, the ingredients in paints and clear coatings are the same: binder, additives and a liquid carrier. Because they do not contain pigment, clear coatings form a film that is transparent, permitting the substrate - perhaps bare or painted wood - to be seen through the coating. Different types of clear coatings create different visual effects, ranging from a satiny sheen to plastic-like reflectivity. All provide some degree of protection to the substrate.

As with paints, clear coatings are available in water-based and solvent-based formulations. Many of these products are designed for interior use, while others are specially formulated for exterior applications. Because of differences in performance, it is very important that you recommend the right type of product for the job your customer has in mind.

Interior Clear Coatings

Clear coatings are used most commonly for interior applications, where the products are typically applied over bare wood or stained wood surfaces, or over a previously applied clear coating. Places they are used include furniture, cabinets, doors, windows, trim, wood paneling, floors, countertops and toys. One, two or even three coats of the clear coating may be applied, depending upon the amount of protection needed, and the appearance desired.

There is a wide range of interior clear coatings to choose from today. Solvent-based interior clear coatings include oils, polyurethanes, lacquers, shellac and alkyd varnishes. Water-based varnishes are also available - binder types include acrylic, modified acrylic, urethane acrylic, and 100% urethane.

Natural Oil Interior Clears

As with oil-based paints, solvent-based natural oil finishes are made with vegetable oils - usually either linseed oil or tung oil - that "dry" or oxidize when exposed to the air. These products actually penetrate the wood to which they are applied and are suitable for most applications to bare or stained wood.

Natural oil interior clears are generally applied just like wiping stains - by brush or rag, then allowed to dry for a short period before the excess is removed with a clean cloth. Application of two or more coats will produce a rich, satiny finish on the wood.

Generally speaking, clear oil finishes should be allowed to dry for several hours between coats. When in doubt, be sure to recommend that your customers follow the manufacturer's suggestions for dry time.

One caveat to the use of natural oil finishes: They should not be recommended for areas that are subject to a great deal of abuse (e.g., floors), or where they would be exposed to water or alcohol (e.g., coffee tables or bar tops).

Solvent-based Alkyd and Polyurethane Varnishes

Alkyd and polyurethane (urethane-alkyd) varnishes are among the most versatile types of clear coatings. An "alkyd" is a modified drying oil that dries faster and harder than the straight oil; "polyurethane" is an alkyd that is modified for maximum toughness and abrasion resistance. These products are available in matte, semi-gloss and gloss finishes. Some come with stain included in the formula in order to eliminate the separate staining step. Common uses range from floors and steps, to trim and furniture.

When working with an alkyd or polyurethane varnish, it is important that the product be stirred very gently - and never shaken - to avoid creating bubbles in the coating. If bubbles are formed, they may not burst promptly after the coating is applied, and may leave small rings or "craters" that can ruin the job.

Both of these types of varnishes are best applied with a high quality natural bristle or polyester brush, although they can also be rolled, using a short nap roller to minimize foam. Brushing should be done very carefully, applying the coating in one direction only, without excessive brushing. Allowing the proper dry time is very important when using polyurethane varnishes. If the dry time is too short (typically, less than a few hours), the second coat can attack the first coat, causing wrinkling or lifting; if the dry time is too long, the second coat may have inadequate adhesion to the first coat. In the latter case, if the first coat has been allowed to dry for more than 24 hours, light sanding will help the second coat adhere properly. Here again, it is critical that you advise customers to check the manufacturer's recommendations on re-coat time.

Shellac

Shellac is a unique clear coating that has been in common use for many years. It is formulated with a binder made from the secretion of the lac bug, which is harvested in India.

The film formed by shellac is quite transparent and glossy. It may be colorless, or have a distinct yellow cast. However, the color of shellac is stable; it will not tend to yellow with age.

Shellac is excellent at blocking stains, so much so that it is used for some stain blocking primers. Since the liquid carrier used in shellac is denatured alcohol, this type of clear coating is quick drying.

In terms of its applications, shellac should not be used for areas subject to alcohol (bar tops - liquor; dressing tables - perfume, cologne). Also, shellac will whiten when exposed to water and some other liquids, which should be taken into account when making recommendations to your customers. A high quality natural bristle brush is ideal for applying shellac.

Lacquer

Lacquer is a term generally used for quick-drying high gloss finishes with excellent clarity. Since lacquers have a low solids content, they typically must be applied in at least several coats. Lacquers are available in sprayable and brushable grades.

The binder most commonly used for lacquer is nitrocellulose, although some lacquers have solvent-based acrylic binders. Lacquers are made with their own type of thinner called "lacquer thinner", which is faster drying and more flammable than paint thinner or alcohol.

If applied over a stain, paint or other finish, there is a risk that a lacquer will craze, i.e., cause cracks, in the pre-existing coating. Due to stricter environmental regulations, lacquers are not as readily available today as in the past, but they are still used extensively by furniture makers.

Water-based Interior Clears

Over the past few years, paint manufacturers have made significant advances in water-based varnish technology. Today, water-based interior varnishes are available in a wide range of products using various binder technologies, including acrylic, modified acrylic, urethane-acrylic combinations, and all-water-dispersed urethane.

Water-based varnishes offer many of the same benefits that are associated with water-based paints, including soap and water cleanup, and low odor when compared to solvent-based products. However, in general, they are not quite the equal of solvent-based clears in terms of resistance properties and very high gloss potential.

Many water-based varnishes have a milky-white appearance, which is quite helpful in applying an even, uniform coat. However, the coating will dry to a perfectly clear finish. After application to wood, some water-based varnishes will be more clear and colorless than solvent-based varnishes, although the latter tend to provide more "warmth" and "depth" due to their amber tone, higher head-on gloss, and penetration, which accentuates wood grain.

Exterior Clear Coatings

Clear coatings are also used on exterior applications, where they impart extra beauty to bare or stained wood. Some of the most common applications are on doors, trim, outdoor furniture, boats, and marine or aquatic equipment. Occasionally, exterior clear coatings are used as a top coat on painted objects to provide greater gloss and a measure of extra protection from the elements.

Unfortunately, Mother Nature is not kind to clear coatings. Without the protective effect of pigment, exterior clear finishes have a limited life expectancy.

The culprit is ultraviolet (UV) radiation from sunshine. Because of the clarity of clear coatings, UV can penetrate down to the wood substrate and deteriorate surface fibers, resulting in release of the coating in a relatively short time. This is particularly so with water-based clears, which are quite transparent to UV.

Solvent-based coatings, such as spar varnishes, are better able to absorb UV, which helps delay adhesion failure. However, this absorption of UV is "sacrificial," that is, the film itself is degraded, so the coating life is still short compared with an exterior paint job. To help forestall deterioration of the substrate under clears, some manufacturers include an additive in these products called a "UV absorber." This type of additive can be quite expensive and, unfortunately, the UV-absorber is used up over time.

In an effort to maximize UV protection, it is wise to recommend that your customers apply multiple coats of any exterior clear coating to give added film thickness. This will render the finish more durable and provide greater protection for the wood substrate.

Moreover, all exterior clear coatings should be reapplied frequently when used over wood - generally, every year or two - even if the coating has not failed significantly. A preventive maintenance coat in exterior applications will add to the life of the job without the need for total removal of the coating if some film failure should occur. Suggest that your customers follow the manufacturer's recommendations in this regard.

Solvent-based Exterior Clears

There are several different types of solvent-based exterior clear coatings. Solvent-based exterior varnishes are typically alkyd or polyurethane products that are similar to the corresponding interior products, with one added ingredient: UV absorber. This additive affords these coatings greater durability and provides an extra measure of protection against adhesion failures on wood. It also helps account for the higher cost of these products compared to interior counterparts.

Spar varnishes, also called marine varnishes, are made with a tung oil and/or an alkyd binder. Out of all the clear coatings, they generally offer the best performance on outdoor furniture, wood planters and, of course, boats, oars, and the like. Spar varnishes also offer excellent resistance to cracking and peeling. In terms of appearance, they have a very high gloss.

Oil finishes are sometimes appropriate for exterior applications, but again, will have a limited life compared to interior use. Have your customers consult the manufacturer's guidelines about the suitability of using an oil finish for an exterior application.

Safety Considerations

To make sure your customers complete their clear coatings projects in a safe manner, it is wise to offer some guidance on the use and disposal of materials.

When your customers are applying solvent-based clear coatings, recommend that they wear proper eye protection; if they are applying these products indoors, also recommend that they work in an area with ample ventilation and suggest that they wear an appropriate respirator.

Be sure to warn your customers about the fire hazards associated with the use of flammable solvents. In this regard, rags and papers soaked with oil, alkyd, polyurethane or spar varnish should never be left in a wad or pile. Rather, they should be spread out to dry in a safe place away from children, pets and heat sources, and then disposed of. Since guidelines for disposal of hazardous materials vary widely, suggest that your customers check the procedures in their community.

Water-based clear coatings should never be poured down the drain. Not only is this practice illegal in many areas, but it can clog pipes. Instead, unwanted finish can be spilled onto scraps of cardboard and allowed to dry away from children and pets. At that point, the scraps can be discarded in the trash.

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