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What is a 'spray to color' stain?
A "spray to color" stain is defined as any semitransparent color material that is sprayed to the desired color without wiping off with a rag cloth.
What are the benefits of a 'spray to color stain' and why should I use it?
First, it is important to understand the differences between a spray to color stain and a traditional "wiping" stain. Throughout the history of wood finishing in the United States, manufacturers have typically used a wiping stain to develop color in their finishing processes. There are two reasons why wiping stains have been the material of choice.
The No. 1 reason is that most finishers consider wipe-on and wipe-off stains easier to use. Secondly, wiping stain materials typically enhance the grain of the wood, which appeals to the American consumer base.
Spray to color stains, on the other hand, have been feared as difficult to apply consistently. Also, some spray stains "mask off" and obscure the material beauty of the wood, resulting in a cheap appearance. However, new technology has practically eliminated these objections. New water and petroleum solvent system formulations allow the spray operator to "layout" the stain for more consistency.
New low velocity, high volume-low pressure (HVLP) spray gun technology offers much more spray operator control of the stain application. These guns eliminate common problems in application of spray stains such as "halos" and "mottling." Advanced spray staining technology often uses colorants that are super finely ground to a level that allows the stains to have an extremely clean look that appears similar to a wiping stain.
Other reasons to use a spray to color stain are:
- To increase productivity by eliminating the labor intensity of a wiping stain application
- To reduce VOC emissions by using a water-based spray to color stain
- To provide a more even stain color on wood species that become blotchy when using a wiping stain; these include maple and cherry
- To achieve deeper, richer colors on hardwoods by using a spray dye stain
Choosing the right stain materials
Choosing the right product for the job is critical for a successful outcome. Stain with dye colorants will provide the most clarity of color and will penetrate deep into the wood. Dyes stain the wood uniformly; however, they do nothing to enhance or bring out the grain of the wood. Dye stains are often used on figure veneers because of their "clear" appearance; they are also used under a wiping stain to provide background color depth. Be sure to select a dye stain that is light fast. Most dyes are susceptible to color change, although synthetic dye stains are the most UV resistant.
Pigmented stains provide a more opaque appearance due to the colorant's ability to penetrate far less into the wood than a dye stain. Pigmented particles in these spray stains will congregate in the pores of the wood and will, to a degree, enhance the open grain of the wood.
Some spray to color stains are formulated with both dyes and pigments to take advantage of the features of each colorant type.
One of the most common problems when using a spray to color stain is selecting a stain with the wrong solvent system. Spray stains are formulated with a blend of solvents that have different vaporization rates. If a stain is drying too quickly, it may be difficult for the spray operator to apply the stain uniformly without overlap marks and inconsistent depth of color. If the stain is drying too slowly, it may puddle on the surface of the wood, causing uneven stain color. Generally, faster evaporating stains work best on hard, dense wood species. Slower evaporating stains work best with softer wood species to allow the spray operator to "wet out" the wood sufficiently.
Selecting and setting up the spray gun
HVLP guns with the lowest particle velocity (the speed of the particles traveling from the gun to the port) are considered the best spray technology for spray stains. It is recommended that the spray operator try several brands and types of HVLP technologies to spray the specific stain material in order to decide which gun will provide the best performance. These guns require a high volume of air to operate at peak performance. Therefore, make sure that the correct cfm (cubic feet per minute) of compressed air is supplied to the gun. If the guns are "starving" for air, the spray to color stain process will be problematic, with less than desirable results.
Gun setup sizes generally range from .035 to .060 in. or an average of approximately 1.5mm. Actual size will depend on whether or not the gun is a pressure, siphon or gravity feed gun, as well as the type of material and the flow rate of the process.
To set up the spray gun correctly, the operator must first determine the correct material flow rate per minute — the amount of coating that is flowing through the gun per minute — for the process to achieve satisfactory results. The correct flow rate will be determined by the desired color, the number of passes of the gun over the part, the gun speed, and the type of spray to color material. If the flow rate is insufficient, the stain will have a dry, grainy appearance. If the flow rate is too high, the material will puddle on the surface.
Normally, the stain should be applied with a flow rate that will not appear glossy and wet or dull and dry. The exception would be a water spray stain. Water spray stains generally need to be sprayed wetter than solvent-type stains.
After the correct flow rate is selected, the operator will need to set the correct atomization air pressure. The normal procedure is to turn the air gun regulator down to 5 psi to begin with. Then trigger the gun while spraying onto a test panel and gradually turn the air pressure up until the fluid stream is broken up into particles of satisfactory size.
The final step in setting up the gun for spray stain is to set and check the flow pattern width. To set the pattern, hold the gun to correct gun-to-part distance from the test panel. Without moving the gun, trigger the gun for a few seconds to form a shot pattern onto the test panel. If the shot pattern is too heavy or fat in the middle of the pattern, open the pattern adjustment wider. If the shot pattern is too wide, resulting in a narrow, starved area in the center of the pattern, simply narrow the width of the spray gun pattern. The shot pattern should appear flat and uniform in width throughout the pattern when properly adjusted.
If the pattern is heavy on the top or bottom, remove the air cap and clean thoroughly, making sure that all of the air passages are clean. If cleaning the air cap does not resolve the problem, return the gun to the manufacturer for repair.
Spray stain application technique
To be successful in applying spray stains, it is very critical that the spray operator uses consistent gun spread. The spray operator must pace himself and "walk" with the forward and backward motion of the gun to maintain consistent gun spread. Gun to part distance is also critical to stain color uniformity. Any gun waving motion will cause disappointing results. Most wrist, elbow, arm and shoulder motions should not be used to move the gun across the part. Good spray technique will require mostly body motions to achieve accurate gun to part distance.
Most spray to color stain processes will require the spray operator to use a 50 percent overlap method to maintain a consistent application of color. This technique requires the operator to overlap the previous pass of material by half of the width of the spray pattern.
We recommend that the spray operator uses "box coats" to apply the stain if the process allows the operator enough time to perform the technique. To box coat, the operator will spray a light coat of stain across the direction of the grain and end up spraying a second light coat with the direction of the grain. If possible, use two box coats to spray the stain to the final color. If using box coats, the stain will need to be reduced in color concentration to achieve the correct color. It is always easier to hit a consistent uniform stain color by using a less concentrated stain color and applying multiple passes. Always fully trigger the gun about 1 in. away from and beyond the edges of the part.
To ensure that the spray operator hits the correct color depth, a step panel will be necessary. A step panel is a visual guide or color standard that has each step of the finishing process exposed to allow the spray operator to visually compare with the work process.
As with all finishing procedures, practice will make perfection. With the right spray gun setup, along with the correct stain materials, spray to color stains can be enjoyable and rewarding to use. Don't let the fear of failure stop you from using spray to color stains. Only after trying these stains will you know what you have been missing by only using a wiping stain.
Ed. note: Phil Stevenson is the founder of the American Wood Finishing Institute in Vineland, N.J.
author: By Phil Stevenson