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Madison, WI 53726-2398
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Title: Mill Glaze: Myth or Reality?

Source: USDA Forest Service, Forest Products Laboratory, FinishLine, 2013

Author(s)Knaebe, Mark

Publication Year: 2013  View PDF »

Related Publications: view

Category: FinishLines

Abstract: Since the mid-1980s, a condition called 'mill glaze' (also called planer's glaze) has sometimes been blamed for the failure of a coating on smooth flat-grained siding and some other wood products. The exact cause of this problem has been a subject of controversy. Many people believe that the coating fails as a result of the planing and/or drying processes. They speculate that the milling or planing process overheats the wood and brings more water-soluble extractives to the surface, creating a hard varnishlike glaze. They attribute overheating to dull planer blades.

An earlier FinishLine ('Why House Paint Fails') described the problem of mill glaze according to reports we had received. We have tried to duplicate mill glaze failure in the laboratory. The tests have included planing lumber with dull blades at high feed pressures. We have been unable to obtain a 'glazed' surface. This does not necessarily mean that mill glaze cannot happen; it means that we have not been able to duplicate it in the laboratory. Although research on 'mill glaze' effect has not continued at FPL, we did investigate a number of reported mill glaze failures. In all cases, the failures were readily explained by other failure mechanisms, including raised grain, degradation of the wood surface by ultraviolet (UV) radiation prior to painting, insufficient thickness of the coating system, improper surface preparation, and moisture problems.

Keywords: mill glaze; planing, surface roughness, surface quality, wood defects, finishes, sanding

File size: 175 kb(s)

Date posted: 07/17/2013

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