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Mill Glaze (on New Redwood and Cedar)

By: 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

A condition known as mill glaze (also called planer's glaze) can occur on smooth flat-grained western red cedar siding and occasionally on other species, such as redwood. There is controversy over the exact cause of this condition, but it seems to be a result of using dull planer blades and is exacerbated by the difficult-to-plane flat-grained surface of the lumber. In vertical-grained wood, the earlywood/latewood bands are perpendicular to the surface. Because of this perpendicular orientation, vertical-grained lumber is easier to plane, even with dull tooling. Planning causes two distinct changes to the surface of the lumber-- it burnishes the surface and crushes some of the wood cells.

During the planing or milling process, overheating of flat-grained siding may bring more water-soluble extractive to the surface, creating a hard, varnish-like glaze. Excess water-soluble extractives can also form on the surface during kiln drying. As these extractives age, particularly in direct sunlight, they become insoluble and difficult to remove. If these extractives occur prior to final planing or sanding of the lumber, the final surface preparation usually removes them. The wood surface can be tested for this glaze 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 of the surface will remove this glaze.

The second factor usually found with the glazing on flat-grained wood is crushed earlywood on the pith-side of lumber. Dull planer blades tend to burnish the surface and crush the less dense earlywood bands directly beneath the more dense latewood bands at the surface. Later, when these boards are exposed to weather, the crushed earlywood absorbs moisture and rebounds, causing the surface latewood 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 and subsequent wetting will continue to cause the surface to deform. Exposing the wood siding to the weather for a short period may help to condition the surface. One or more wetting and drying cycles are necessary to remove these planer-induced stresses in the wood. However, wood should not be exposed to the sunlight for more than 2 weeks before application of a film-forming finish, because this 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. The rough side is the side of choice for application of penetrating semitransparent stains. Solid-color stains form a film and will give a much longer service life when applied to the rough-sawn side. In addition to the lack of mill glaze, the rough side has two other advantages. The film build on the rough side will be greater, and the film will have greater mechanical adhesion or "bite". The best film build is obtained by brush application. If the finish is applied by roller or spray, it is advisable to back-brush the stain immediately following application to even out the finish and work it into the surface of the wood, thus avoiding 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, then allow 4 to 5 days for the surface to dry before finishing. Scuff sand the surface of the wood with 50-80 grit sandpaper prior to finishing.