If you do any kind of wood staining you will run in to mill glaze. It will effect how the stains you use penetrate, more often than not I get the call after the homeowner has tried to do it themselves and made a mess of things that they want me to fix. The article relates this primarily to cedar and redwood but I see it with other types of wood like white pine or spruce too. I first saw this in Powerwash.com so I am not trying to steal anyone else’s thunder here. It’s a bit long but worth reading. Hope it helps some of you.
A SCIENTIFIC EXPLANATION OF MILL GLAZE
Mill Glaze (on new redwood and cedar)
R. Sam Williams and Mark Kanebe
Chemists
Wood Finishing Research
USDA Forest Service
Forest Products Laboratory
One Gifford Pinchot Drive
Madison, Wisconsin 53705-2398
October, 1995
The condition known as mill glaze (also called planer’s glaze) can occur on smooth, flat-grained western red cedar siding. It can also occur occasionally on redwood and other species. There is some debate over the exact cause of this condition, but it seems to be a result of using dull planer blades in the milling process. Exacerbating the condition is that the flat-grained surface of the lumber is difficult to plane. When the surface of the lumber is planed, two distinct changes occur to the surface – the surface is burnished and some of the wood cells are crushed.
During the milling process, overheating of flat-grained siding may bring more water-soluble extractives to the surface, creating a hard, varnish-like glaze. Additional water-soluble extractives may also form on the surface during kiln drying. As these extractives age, particularly in direct sunlight, they become insoluble and difficult to remove. If this occurs prior to final planing or sanding, this final surface preparation removes them. Test for mill glaze caused by extractives by carefully placing a few drops of water on the surface prior to finishing. If the water beads and resists soaking into the wood, the surface probably has mill glaze. Light sanding will remove it.
The second factor that contributes to mill glaze on flat-grained wood is crushed early wood on the pith-side of lumber. Dull planer blades tend to burnish the surface and crush the less dense early wood bands directly beneath the denser late wood bands at the surface. When these boards are exposed to weather, the crushed early wood absorbs moisture and rebounds, causing the surface late wood bands to rise.
These two surface defects act in concert to cause flaking of the finish parallel to the grain. The pith-side of flat-grained lumber finished with a single coat of oil-based solid-color stain is particularly susceptible to his type of finish failure.
Sanding will remove the extractives build-up, but it is not likely to remove all the crushed wood. Subsequent wetting will continue to cause surface deformations. Exposing the wood siding to the weather for a short period may help to condition the surface, and one or more wetting and drying cycles may be necessary to remove the planer-induced stresses from the wood. However, wood should not be exposed to the sunlight for more than two weeks before applying a protective finish, as excessive exposure decreases the adhesion of the coating.
The simplest and best solution to the problem of mill glaze, when using flat-grained bevel siding, is to install the siding rough side out, which is the side of choice for applications of penetrating semi-transparent stains. Solid color stains form a film and provide a longer service life when applied to the rough-sawn side. Besides lacking mill glaze, the rough side has two other advantages: The film buildup on the rough side is greater, and the film has mechanical adhesion or “bite”. Brushing delivers the best film buildup.
If applying the finish with a roller or sprayer, it is advisable to back-brush the stain immediately to even out the finish and work it into the surface of the wood. This prevents bridging, gap formation, and lap marks. If the flat-grained siding must be installed smooth-side out, remove the planing stresses by wetting the surface. Allow it to dry for four or five days before finishing. Scuff sand the surface of the wood with 50-80 grit sandpaper prior to finishing.
