There are two types of adhesion that occur in coatings application: chemical bond and mechanical bond. A mechanical bond occurs as the result of sanding. A chemical bond occurs when the new finish coat re-melts the previous coat allowing both to intermingle and become one. That type of bond occurs only inter-coat.
Equally important is the need for a strong bond between the coating and the raw wood. Over the years, much has been said about things that degrade or cause the coating to not bond to the wood. Pitch pop in fir is one. Another is the application of finishes to oily woods such as teak. A third, and probably the most common, would be the over sanding of hardwoods, such as maple. Doing so can create a surface so slick and/or impervious that the coating has nothing to attach to.
There are ways to deal with this as a prophylactic measure. A “sticky” sealer can be applied to aid the bond between the raw wood and the coating. In the case of oily woods, it is recommended that a wipe-down with lacquer thinner be done to remove surface oils. And again, don’t over sand, but give the coating some “tooth” to hold on to. With maple for example, I like to sand no finer than 120P.
How Milling Affects Gluing
We have no such things to aid glue’s bond to wood. Other than the lacquer thinner wipe-down, wood glue is on its own to bond to the wood and create a bond that “can be” 50% stronger than the wood itself. That bond is the result of a mechanical action before the glue is applied. But Gene Wengert, says that the longer the glue joint surface is exposed to the air prior to application of the glue and the clamp, the more likely it is that moisture will affect that surface and diminish the bonding qualities of the joint. After a week, the affect is measurable.
Freshly milled lumber should be flat and straight. However, he notes, the longer that the newly milled surface is exposed to the air the more that the wood cells will move and change shape to the extent that surface pores swelling up will result in warpage and will affect the bond between surfaces. And, just as importantly, that swelling will reduce the surface’s ability to accept the glue and create the best possible bond. It only takes a few thousandths of movement, and only a few days, for the glue joint to become inferior to one that was milled and then quickly glued and clamped.
The finisher also needs to be concerned with those swelling surface cells reacting with the moisture in the air to make the surface less willing to accept the applied finish.
As I’ve discussed the problems of developing strong coatings and gluing bonds, it occurs to me that I’ve made a heck of a case for using water-based stains and finishes. Specifically, using these materials on a project that has not been quickly built and finished may have some pluses to it. But that is a subject for another day.
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