Blotching turns our beautiful wood into something less attractive than we would like. The “B word” is pretty evocative; maybe it would be good to understand how it happens. Blotching occurs when a wipe stain is applied and the wood takes the stain unevenly. Why???

Under a microscope, wood looks like millions of drinking straws glued together. These straws run this way and that along the length of the tree, carrying nutrients where they need to go, and forming interlocking fibers that give the tree strength to grow ever larger.

When that great big bundle of straws hits the saw mill, the sawyer’s job is to turn it into marketable lumber. Sawyers do that by turning something long, cylindrical, and uneven into something square and straight. Square and straight are rarities in nature. Those are human passions…not Mother Nature’s.

Free Webcast: How Finishing Can Grow Sales

Finishing guru Bernie Bottens focuses on versatility in finishing and coating to help you venture into new markets and grow business, drawing tricks from traditional techniques and new finishing advances.

Webcast Date:
December 5, 2012


Time:
10:30am CT/11:30am ET

Within the sawn lumber those straws come to the surface of the board and abruptly end where the saw cut them off. The ends of the straws are exposed there. But just inches away may be another area in which the straws run parallel to the surface of the board, with fewer straws having been torn open by the saw.

As Sawyers Cut Wood Fibers

Let’s say that we have taken that lumber and turned it into a table. Now, let’s stain the table top. The wood will accept that stain in different ways depending upon which way the straws intersect the surface of the board.

Remember learning about capillary action in biology class? Capillary action is fully operative here. The stain will more readily enter the straws that have been torn open. The proof of my point is seen in the end grain of the board, where stain color is always most intense. Why? Because the highest concentration of ends of straws is found at the end grain.

Remember learning of osmosis? Osmosis is necessary in getting the stain to go through the wall of the straw. That’s a tougher go for the stain. The avenue of least resistance is via capillary action. The avenue of greatest resistance is osmosis. More color will enter the wood through capillary action than through osmosis. Again, that’s why the end grain of the board will always be darker than the faces.

Blotching is going to occur in areas where the grain allows for the stain to enter the easiest. Alder is a good example of a tough wood to stain without blotching. It grows fast, with a lot of internal stress in the wood, which is generally soft and porous. The straws within it twist and go wildly about to create strength to support that fast growth. That wild grain means lots of areas where the ends of the straws intersect the surface of the board.

There are a number of means by which we can reduce blotching. White wood sanding is the first. The finer the grit that you sand with, the more the grain is closed. I prefer not to over-polish wood that is to be stained. But in the case of alder, I like to finish up with a finer grit. I do that to help reduce blotching. I use 220 grit on alder.

Applying a pre-stain conditioner will also work. Use a clear stain base with no pigments or dyes added. There are lots of recipes for conditioners. Slather it on, let it penetrate, and re-wet those areas where holidays appear. Then, wipe it off. Some like to let it dry. I like to come right back over the top with the stain I am going to use. The pre-stain conditioner goes into the straws and fills them. Having been filled with the conditioner, there is less room for the color when we add it and a more homogenous look should result.

You can reduce blotching with a wash coat. A wash coat is a clear finish that’s been greatly reduced in solids by the addition of thinner (usually 5 – 10 percent volume solids are the effective strengths). Spray that on, let it dry, scuff it very lightly, then stain.

The down side of these techniques is “holdout,” since the effective strength of the stain is reduced and the intensity of the color is lighter because something has been done to alter the wood’s ability to accept the stain. Have your tint specialist create a custom stain with a tint load 1.5 to 2 times higher to compensate for the holdout. Until next time…spray on!

Have something to say? Share your thoughts with us in the comments below.