Stain blotching-continuing discussion
Bill Tindall, E.Tn.
There have been recent discussions of this effect and its mitigation with wash coat or conditioner pretreatment before stain, which works if properly done. It may be that surface preparation is an element not considered in the previous discussions.
Lets agree on a definition so we will be talking about the same phenomena. "Blotching"- undesirable (in the eye of the beholder) unevenness of depth of shade after staining. Blotching is not a color variation, at least for this discussion.
There are at least three causes and at least my response to them is different.
1. Torn grain- Torn grain makes fractures that extend into the wood which absorb stain more than surrounding undamaged wood. The cause could be machine or hand tool surfacing that lifted wood fibers rather than cut them. Or it could be fibers lifted by a coarse grit of sand paper or even a scraper.
I think we would all agree that blotching from this source should be eliminated by proper surface preparation before stain. These defects are readily spotted by an experience eye when finish sanding. Torn grain traps sanding dust and makes these areas look dull compared to surrounding undamaged wood. I work diligently to eliminate torn grain as a source of blotching.
2a. Grain orientation, hand planing- Done perfectly, hand planing slices through wood fibers without distorting the fibers left behind. When planing into rising grain it may be impossible to not alter what is left behind compared to planing falling grain, at least as far as staining is concerned. In my hands, a hand planed surface enhances the shade differences of a surface with grain orientation changes. I can not stain my hand planed surfaces to my satisfaction. They become overly dark and undesirably blotchy. Perhaps conditioner would make things well but I have never tried it in this situation.
2b. Sanded surfaces- With small grits I think sanding compacts wood fibers. My well sanded surfaces take up stain where there is rising grain (right side up U's) more than falling grain (upside down U's). However, with my sanding technique, the difference in stain take-up between these grain orientations is not extreme. I can stain walnut and cherry and achieve an attractive result to my satisfaction. The grain changes are apparent but too apparent, and they add interest. I find no need to use conditioner or wash coat in this situation.
3. Crotch wood- Some grain in crotch wood is standing on end. At the microscopic level this surface looks like looking at the end of a stack of soda straws. These areas will drink up stain and go exceptionally dark. I always wash coat crotch wood before staining.