A couple of weeks ago I brought up the dreaded “B word”. Blotching is something that we woodworkers love to hate. It 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.
In a nut shell, it occurs when a wipe stain is applied and the wood takes the stain unevenly. Why???
Let’s take a microscopic look at wood. When we do, it looks like millions and millions of drinking straws all glued together. Those straws go this way and that along the length of the tree to sustain the tree’s reason for existence…to grow. These straws carry nutrients to where they need to be and those interlocking fibers give the tree the strength it needs 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. And lest we forget, square and straight are rarities in nature. Those are human passions…not Mother Nature’s.
The result within the sawn lumber is that we find those straws coming to the surface of the board and abruptly ending where the saw cut them off. The ends of the straws are exposed there. 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.
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 Biology class? Remember the concept of capillary action? That concept is fully operative here. The stain will more readily enter the straws that have been torn open. The proof to my point is the end grain of the board. Where will the stain color always be the most intense? Where the highest concentration of ends of straws is found — the end grain.
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Remember osmosis? That’s another biological concept involved here. 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 or 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. Remember my recent comments about alder? Alder is a tough wood to work with from the standpoint that it easily blotches. I learned cutting alder for firewood that alder is more like a giant weed. It grows fast and there is a lot of internal stress in the wood. At the same time, the wood is generally soft and porous. The straws within it twist and go wildly about to create strength to support that fast growth. The woodworker’s lament to alder is found in that wild grain which means lots of areas where the ends of the straws intersect the surface of the board.
All that having been said, there are a number of means by which we can reduce blotching. But again, I wouldn’t be Bernie if I didn’t bring up my Bernie-isms. Here goes. There are very few silver bullets in wood finishing. But there are some pretty safe bets.
White wood sanding is the first. The finer the grit that you sand with, the more the grain is closed. As I said last week, I prefer to not 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. Usually that is simply a clear stain base with no pigments or dyes added. That being said, there are lots of recipes for conditioners. Slather it on, let it penetrate, and rewet 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 amore homogenous look should result.
Awash coat will also reduce blotching. A wash coat is a clear finish that has been greatly reduced in solids by the addition of thinner. Usually 5 – 10% volume solids are the effective strengths. You spray that on, let it dry, scuff it very lightly, and then stain. Again, the wash coat enters the straws and fills them up.
The down side of these techniques is “holdout.” 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. As long as you are working with a stain for which you know the recipe, that’s no big deal. Have your tint specialist create a custom stain with a tint load 1.5 to 2 times higher to compensate for the holdout.But if you don’t have that information, then another set of issues can show up that can have a negative effect on your finishing.
We’ll go into the pitfalls of adding color when the wipe stain isn’t dark enough next week.
Until next time…spray on!
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