Mill Glaze: Myth or Reality?

finishline_graphic_4webSince 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. This newly revised Finishline tackles the tough question of Mill Glaze: Myth or Reality?

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. Tests trying to duplicate mill glaze failure in the laboratory have been inconclusive. Although research on ‘mill glaze’ effect has not continued at FPL, a number of reported mill glaze failures have been reviewed. 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.