Author: Phil Stevenson
We often hear finishers comment that they never use dyes or dye stains in their wood coloring development process. In this article, we want to present the use of dye stains in order to help finishers realize their potential to produce quality products using dye stains. The minimal utilization of dye stains in the US is significant, especially compared with European manufactures that use a high percentage of solvent or water based dye stains. Why don’t finishers use more dye stain technologies here in our country? In analyzing why they are not more widely used, several considerations must be looked at.
1. First, our customer base will often not accept the results of the appearance of the product using a spray to color dye stain as they do in Europe.
2. Second, many finishers have successfully used a pigmented wiping stain to satisfy most of their customer requirements and do not have the knowledge of where and how to use dyes to provide a deeper, richer looking finish.
3. Third, many finishers stay away from using dye stains because of difficulties in the spray application process to achieve consistent results.
4. There are, however, many good reasons why you may want to utilize dye stains, especially since it can be easy to apply these products.
Dye stain composition and characteristics
The two categories of colorants that are used in the wood industry are dye colorants and pigment colorants. In comparison, there are several significant differences between the two types that we will consider.
- Dye stain colorant particles are very small in size versus pigment particles, which are much larger.
- Another difference is that pigments, used as a colorant in wiping stain, always settle to the bottom of the stain vehicle, while dye stain colorants always remain suspended in the stain vehicle.
- Due to the physical size of the dye stain particles, they penetrate very deeply into the pores of the wood, giving a much darker and cleaner appearing color than may be achieved using a pigmented wiping stain
- Pigment stains remain high on the surface of the substrate and may cause a muddy appearance that will not occur with a dye stain.
- A wiping stain will also tend to wipe off easily from the tight grain and will not develop as dark of a color as can be achieved with a dye stain. However, a dye stain does not provide grain contrast or strikeout to darken the open pores to enhance the natural beauty and grain characteristics of the wood.
Typically dye stains have been susceptible to color change and fading due to sunlight ultraviolet radiation. For example, a red tone dye may dramatically change color to a yellow brown tone. New dye colorant technologies, called aniline dyes, are much more UV stable. They are manufactured with petroleum products instead of natural substance such as walnut husk. In many of the new European technologies, synthetic dyes are blended with super finely ground particles that are very lightfast and do not bleed or re-wet when applying the subsequent coat in the finishing process.
There are also several different types of these dye technologies that are available for the wood finisher to choose from and are classified by the solvent in which they dissolve.
- Dyes that dissolve in water
- Dyes that dissolve in alcohol
- Dyes that dissolve in lacquer thinner or other strong solvents and are called oil soluble dyes.
- Non grain raising (NGR) dye that dissolves in glycol either.
For production finishing and for overall flexibility of the product, I generally recommend a NGR, pre-metalized, liquid dye concentrate. This type of dye can be reduced with acetone, lacquer thinner, alcohol, or water.
When and where to use dyes
There are many reasons that a dye stain may be a better choice than a wiping stain.
- A dye stain is a very logical for use on fancy veneers and inlays.
- Dyes will not obscure the grain or inlays as would a pigmented wiping stain.
- Dyes should be used when a very uniform color of finish is desirable.
Many dye concentrates can be used in a variety of finishing materials. They can be used to make a dye stain; added to a pigmented wiping stain to strengthen the stain’s depth of color; or added to a shader or toner to uniform and change the natural color characteristics of the wood substrate. Adding a dye to a shader or toner is a good choice to maintain clarity of the color of the finish.
Dye stains also may be used in conjunction with a wiping stain. In many high end finishing schedules, a dye stain is used prior to applying a wiping stain. In this scenario the best of both stain characteristics can be utilized. A dye stain is applied to the wood as a first step to develop depth of color. This step will also help uniform the color tones of the wood. With some dye stains, a wiping stain can be applied directly over the dye stain. Other dye stains need to be wash coated with a highly reduced sealer to prevent lifting or bleed up of the dye color when applying the wiping stain. When formulating dye stain for use under a wiping stain, there are two different objectives to consider. One may be to just darken the overall color of the stain development process. The other goal may be to formulate a contrasting dye color to apply underneath the wiping stain. The result will add life and depth of color to the finish and is especially desirable when using dark stains to prevent the color from becoming too dull or dead. Using bright reds or yellows in the dye stain are most commonly used when achieving this effect.
A very different dye stain effect may be achieved by adding water to a water reducible dye on species of wood such as maple. The water will swell up the pores of the wood and will allow the stain to penetrate the wood in a different manner than a solvent based dye stain. Water will stay open longer than solvents and may allow the finisher to wipe the stain desired color.
Using dye stains on hard species of wood will allow the finisher to deepen the color of the stain to a much greater extent than is possible when using a pigmented wiping stain. Dye stains are also useful to help uniform the blotchy uneven stain effect that is problematic in maple and cherry.
Application of dye stains
Some finishers apply water soluble dyes with a cloth or a brush. However, in general for production finishing, we recommend spraying the stain “to color” without wiping it. Dye stains have always been considered difficult to spray and require a highly skilled spray operator to achieve consistent results. Some of the problems are: shadowing in corners and profiles, mottling of the stain, and uneven depth of color. With newer spray gun technologies that are now available to the industry most of these issues have been addressed. In general, an HVLP gun with the lowest forward particle velocity applies dye stains most effectively. These recently developed new spray gun systems require much less skill on the part of the spray operator. It is recommended to try different guns from different manufactures to identify the gun technology that will provide the best results for your process. A .035 tip size is recommended for spray gun setup. Always use the lowest air pressure (PSI) to achieve the desired aesthetic results in the stain application process. It is highly recommended to use an air cap pressure gauge to measure and verify the air PSI at the tip of the gun. This pressure at the tip should generally range from 4 -7 pounds and never exceed 10 pounds. The material flow rate should also be measured. This flow rate should be checked, set, and maintained throughout the staining process. Minor fluctuations in air pressure or flow rate will change the color of the stain significantly and should be avoided.
Even with improved spray gun technology, best results will still be achieved using basic spray operator techniques. Always maintain uniform gun speed, gun to part distances, proper gun angle, and overlapping techniques.
Other issues, such as mottling, puddling, tiger striping, or uneven color of the material can generally be attributed to the incorrect solvent system used in the dye stain material. Slower solvent blends must be used on soft wood. This will allow the surface to wet out the wood sufficiently to provide consistent color of the stain. On hardwoods, a fast solvent system may need to be used to prevent puddleing and runs in the stain. If your stain system does not provide the results that you expect in appearance, or are not user friendly to apply, you should consult with your coating vendor for a different formulation to correct the problem.
Using dye stains may open up a whole new dimension in your finishing process and provide results that you thought were impossible to achieve. Bring in the dye stains and let’s start finishing!