In an experiment some years ago, when I was first learning about what stains really were, a co-worker and I created some sample boards — small panels with the complete finish applied to them in various colors to demonstrate the colors of the stains with which we were working. These were all pigment stains from probably three or four different manufacturers. We used several different species and cuts of wood and sanded them with the same sandpaper.
We shared the chore of staining over a period of several days and then sprayed sealer and topcoat. The stains were in pint, quart and gallon cans. The pints and quarts were shaken by hand, while the gallon can had to be hand-stirred to get the “mud” off the bottom and then with a hand drill and paddle-type agitator.
At the time, I was surprised by their inconsistent appearance. What was more puzzling was why. Some had been stained on different days by different people, but panels of the same species had also been stained on the same day by the same person. The inconsistency was also present if one person stirred and applied the stain or if the duties were split.
The eureka moment
After many years, I finally have several explanations:
• Different species and cuts of wood have different topography (the shape and nature of the surface).
• Sanding affects the shape and different technicians might sand a bit differently.
• Atmospheric moisture or humidity affects the shape so that a change in the surface can occur as moisture is absorbed or given off. The wood’s moisture content also affects the ability of the surface to absorb the liquid portion of the stain and thus the way that the pigment colorant of the stain adheres to the surface.
• The pigment concentration in that portion of the stain that is being used can vary according to the degree to which the pigments have been dispersed throughout the liquid volume of the container.
• Pigments tend to settle out at different rates and to a different degree of caking or settling hard on the bottom of the can.
• Different pigments have a greater or lesser tendency to agglomerate. Hand stirring might not generate sufficient shear force to break these agglomerates apart.
• Different pigments are of varying particle size. Smaller particle sizes are easier to suspend in a colloidal fashion and will have a more powerful coloring effect.
• There are differences between the pigments used in stains. A high-quality wood stain has a finer grind of pigment, typically with less binder (the resin that causes the stain to stick to the wood) and fewer pigments of different colors. A house or deck stain will have larger particles and more filler or extender pigments, offering increased coverage and hiding at the expense of clarity.
The sealer and topcoats also played a role in the inconsistencies. Sealers have solids such as zinc stearate and amorphous silica that tend to settle. Amorphous silica is also used as a flattening agent in topcoats to lower the sheen, while zinc stearate is used as a sanding aid in some sealers.
With a satin lacquer that contains some amount of silica, if it’s not adequately agitated, the upper portion of the can will not contain the requisite amount of silica to produce the sheen advertised and desired. But lower down, there will be a greater and greater concentration of the flattening compound, causing an increasingly lower sheen. A flatter or lower sheen topcoat will cause the color to look whiter and less clear.
Rules to follow
Here are some good general rules for maintaining a consistent color and sheen:
1. Become very familiar with the finishing products that you use by reading the manufacturer’s literature, including the material safety data sheets, product data sheets, and any instructional material available.
2. Educate yourself through the various media available. Books, videos and articles by recognized experts, Internet discussions and Google searches can all be very helpful. But be careful of uninformed opinion. You will shortly be able to recognize who knows what they are talking about.
3. Store the materials appropriately. For example, don’t allow water-based products to freeze. Avoid very high temperatures that shorten the shelf life of many materials. Note expiration or use-by dates and mark products with the date purchased.
4. Read the directions. Product use information is normally found on the label; more detail can be found on a product data sheet.
5. Use the products at reasonable temperatures. Watch the humidity and be aware of the effect it has on the product’s performance.
6. Shake, stir or agitate well. Stains generally cannot be stirred too much. When using mechanical agitators, try not to whip the product into a froth, especially water-based products. But use enough energy to develop the shear force necessary to break up the agglomerates and distribute the solid particles evenly. If you see bubbles in the liquid, allow it to sit for a while. Most of the bubbles will rise and burst. Low temperatures inhibit this release of the bubbles from the liquid. Hand-stirring most pigmented stains, especially if they have been on the shelf for a while, is typically not sufficient. If you find after agitating that there is still settled material in the lower corners of the can, you need to scrape that loose and then stir again with some vigor. Most pigmented wiping stains cannot be sufficiently stirred by hand or even shaken to break up the agglomerations.
7. Try to sand, stain and apply a first coat of film former (sealer or topcoat material) in the same day. Sanding on one day, staining the next and sealing on the third allows time for atmospheric moisture to change the surface of the wood. A more open or fuzzy surface holds more of the pigment particles, causing a darker or stronger color to appear.
8. Sand consistently. The sanding scratch holds pigment particles. A coarse scratch pattern will hold more pigment, a fine scratch pattern less.
9. Sealers and topcoats should be agitated often when in continuous or frequent intermittent use, such as when spraying a set of kitchen cabinets or chairs and a table, to avoid the aforementioned variation in sheen.
Wood is valued in large part for its pattern, figure and rainbow coloration. Controlling the color to keep it within certain parameters is largely a matter of controlling the materials and processes used in finishing it.
This article originally appeared in the December 2011 issue.