Wood finishing 101: Troubleshooting tips & lighting issues
By Robert Cox Jr.

Wood finishing is one of the last processes in the manufacture of furniture or cabinets and the first thing seen on the retail floor. The wood finish emphasizes the mission of the furniture, cabinets and casegoods with respect to their look and feel.  A pleasing finish can do much to offset a mediocre design, while in contrast, a mediocre finish will detract from well-designed and constructed products.

Up to 90 percent of the entire costs associated with the manufacture of furniture or cabinets apply to the white room, and only 10 percent to wood finishing and its components.

Some basics for white room wood preparation include:

The moisture content of the wood coming from the white room must be consistent.  This allows the wood substrate and the finish to “breathe” together.

The sanding procedures are primarily dictated by the porosity of the wood pieces.  Soft, porous woods, such as pine, requires aggressive sanding and a coarser paper grit.

Overly aggressive sanding (burnishing the wood) on harder woods such as oak, cherry, maple, birch, and alder prohibits proper finish adhesion.  The wood finish may flake off in its entirety on woods with little pore definition.  When manufacturers are using aggressive sanding procedures on harder wood, we recommend a fine mist of water or alcohol to open up the wood to better accept the finish, and the wood finish adhesion will be improved.

Using too fine sandpaper in final sanding prior to finishing will cause colors to be lighter in comparison to the standard.  Worn out sandpaper and/or varying grits of sandpaper change the color absorption of the wood, and cause adhesion issues.

Dark colors are especially susceptible to sanding and adhesion problems and the overall integrity of the wood finish. Whites are also susceptible to adhesion problems as high concentration of pigments (high PVC) are often used to obtain the look wanted. If the pigment concentration is too high and goes above the CPVC (the point at which there is just sufficient binder to cover the pigment surface as well as all the interstitial spaces between the pigment particles in a close-packed system), adhesion becomes an issue or in more simple terms:  basically too much pigment causing adhesion issues.

With that in mind, here are some execution variables and  recommendations:

Proper maintenance of equipment for finish consistency cannot be overemphasized.  The spray equipment must be kept clean and in good shape.  Spray gun tips (orifice) will wear and as they become larger, inconsistent finishing will result.

All excess stain should be wiped off uniformly to prevent adhesion problems and color inconsistencies.

Heavy-bodied stains must be dried properly before the next coat is applied in order to reduce the threat of bubbles and blisters.

After wiping and drying, the stains should be brushed or blown off to remove any excess rag fibers, lint, or dust.

All clear coats, washcoats, sealers, and topcoats should be applied at wet film levels, to obtain the best finish appearance.  As a rule, we recommended 3 to 4 wet mils for low-volume solids.

It is recommended the sealer application be kept closer to 4 wet mils and topcoats closer to 3 wet mils.  In faster manufacturing situations, this helps to prevent blisters, when the finish is exposed to heat.  Millage control of the sealer and topcoat application as recommended by a finishing supplier should be monitored carefully.

Pieces should be examined for wood finishing defects as early as possible, no later than the sealer application stage and prior to topcoat applications. It is easier to adjust sealers than rectifying poor build or poor color in the topcoat step.  Sealers can be tinted and - depending on their formulation - sanded in preparation for the final topcoat.  Therefore, catching finishing problems before the topcoat application is a rule that would be good to follow.

It is recommended that finishing materials be properly warmed before application especially in cold climates.  Checking for flow properties is important for all wood finishing products, especially clear coats.  If the materials are cold, the viscosity will be high and the quality of the finish will be compromised; if the finish is too warm, flooding of the finish (leaving puddles during application may occur.

Finish inconsistencies/troubleshooting         

The following are some of the inconsistencies that can relate to improper product and or execution controls:

Bleed -through: Alcohol-based dye stains can react with catalyzed sealers and topcoats causing a bleed-through that is readily apparent in the finish - usually red or green.

Drying issues: The process must allow the finish to dry and cure from the substrate up rather than from the topcoat down.  This quality check cannot be overstated. In addition to having good air make-up in the room, the air in the ovens must be kept clean or the finish may be affected.

Ovens overloaded with solvent vapors can cause the finish to cure with a greenish-yellowish cast due to the high acidity of the atmosphere in the ovens and cooling systems.  This makes it necessary to wash off and re-finish the pieces.

Improper cure also can create blisters, and bubbles in the finish, leading to poor finish adhesion, and results in a rough feel to the final piece.

Additionally, improper formulation of the topcoat can result in a “wavy cold gravy” appearance. The solvents may be flashing off and drying too quickly.  The solution to this problem is to correct the solvent blend and/or add more flow agents. If the topcoat cures topside down rather than substrate up, this is another reason the wavy, cold gravy appearance will also appear.  Makeup air and oven temperatures should be addressed for correction.

Blushing of the wood finish. This occurs mostly in multiple-step furniture finish applications and is caused by moisture-laden air, usually from high humidity, causing a “white blush” appearance on the wood finish.  The appearance generally comes from the topcoat causing the stains and glazes to “blush up” through the topcoat.

Blushing can be corrected in many ways, Reduction of air flowing through the finishing spray guns is recommended, along with the use of a slower solvent to allow the finish to dry from the bottom up, not topcoat down,  Again, keeping the finishing room dry without excessive humidity is an important aspect of controlling this execution step.

Lifting of the finish: In catalyzed wood finishes, if the catalyzed sealer is not cured properly in conjunction with the curing of the catalyzed topcoat, lifting of the wood finish occurs. This lift gives the finish an alligator-skin-like appearance, ruining the pieces and requiring costly clean-up and re-finishing.

Wood finish checking: A wood finish will check if the finish sets up quickly and drying is too fast.  This happens in the topcoat step with both nitrocellulose lacquers and conversion topcoats.  A finish will fail the ASTM cold check test if the topcoat is too hard and lacks flexibility, or if the operator is putting too much material on the pieces (in excess of 3-4 wet mils per topcoat application.)

Veneer checks:  Veneer and finishing checks are sometimes confused.  The visible difference is the consistency of the checking pattern throughout the veneer.  A finishing check is much more random in appearance.

Trapping bad air/poor drying:  As parts move from finishing station to finishing station solvent- laden air may get trapped in the finish.  This will impede the cure and if the solvent-laden air is not replenished, the pieces will become “gummy”, causing sand-throughs, poor adhesion and rub-throughs.

Pore bleed back: Wiping stains, fillers and heavy glazes may bleed back through the pores of the more porous woods such as oak due to improper air. This finishing problem usually results in a costly refinishing process drying and lack of clean make-up.

Fish-eyes/craters: The cause of the defect comes from two areas:

External: Lubricants and grease used for equipment maintenance often contain silicone, which can contaminate the finish causing the fish-eye/moon crater problem.  The silicone can also come from hand creams, soaps, and deodorants used by employees.

Internal: Many clear wood finishes are formulated with some silicone (very small additive weight) to enhance the slip or feel of the finish.  If the wood finish is formulated with too much silicone, or if the manufacturing process too much silicone is added, the fish-eye moon crater problem appears.

Strange as it may seem to many, one way to remedy this problem is to use what we call a fish-eye/moon crater corrective additive which has as its basic formula some solvent and silicone – yes silicone (slip agent) – the very product that contributes to the problem.  However, this additive is made under controlled conditions and is different in its mixtures from the silicones found in other products.

All manufacturers should check their MSDS from their suppliers of lubricants and greases to ascertain the extent of silicone in the product.  Silicone is very “flaky” both figuratively and literally and becomes airborne easily,  Thankfully, wood finishing suppliers are aware of this problem and formulate the finish to be as forgiving as possible.

Different sources of lighting can impact the color of the finish.  Pictured left to right are renderings under daylight, incandescent and fluorescent lighting.

The Importance of lighting

Colors appear differently under different light sources. Light booths can be used to simulate the different lighting conditions in a room, and help reduce project rejections. These booths offer the possibility to inspect wood pieces under different lights such as daylight, incandescent light, ultraviolet light, department store light, etc.

Colors should be matched under daylight, which gives a true view of the color and its hue, chroma, and lightness. From a wood finishing standpoint, the following points for matching colors must be observed: 

Incandescent lighting shows colors to be “a touch” warmer and redder than in daylight. In showrooms, the lighting is primarily a combination of incandescent lighting with some diffused daylight.

Fluorescent lighting (with windows supplying some diffused daylight) is the most commonly used type of lighting in manufacturing facilities. While the least expensive, it is the worst light for color matching. Applied under fluorescent lighting conditions, the finish will look green or yellow with a washed out appearance.  While the reaction would be to strengthen the color and make it warmer, if adjusted it could appear too strong and/or too red in a home or retail store.

To ensure consistency, choose a finished piece as an acceptable standard under daylight conditions and sign off on it. Keep in mind brown tones may look slightly warmer or cooler upon aging, so samples should not be kept longer than 6 to 9 months.

Wood-finish suppliers and manufacturers design and execute the finish so that quality and integrity is not compromised.  After selecting the products for the finish, the parameters for the execution should be established in tandem.

Source: Robert Cox Jr. is the former president and CEO of The Gilbert Spruance Co. and Spruance Southern Inc. with 40-plus years of industry experience. He lives in Oreland, PA; e-mail: [email protected].


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