Mill Glaze.... Myth or Fact? Why is it SO Shiny?

By Bill Christou, 10/16/17

Installing and protecting your new deck correctly from the outset will ensure enjoyment for years to come while preventing the need for premature deck refinishing, repair or replacement.

The condition called "mill glaze" (also called planer's glaze) has frequently been blamed for the failure of a coating on decking, siding and various other wood products. This failure of the coating can potentially lead to problems such as cracking, raised grain, mildew growth and wood rot.

Some contractors claim you should begin the process of deck protection by refreshing new surfaces with a mild surface sand to remove so-called "mill glaze" while opening up the grain.

The exact cause of "mill glaze" has been a subject of controversy. Many believe that the coating fails as a result of the planing and/or drying processes. They speculate that incorrect milling or planning of boards overheats the wood. The overheating of boards is usually attributed to dull planer blades and the claim is that this overheating opens the pores and actually brings water-soluble extractives to the surface, creating a hard varnish-like glaze.

The remedy for this "mill glaze" is to gently re-surface the boards with a light touch sanding which will clean and brighten the wood while allowing for the opening of the wood's pores. This is important because it will thus assist in the absorption of an oil-based finish to help in the prevention of algae, mold, and mildew attacking your deck boards.

In a 2013 article Mark Knaebe of the USDA Forest Products Laboratory noted that he had tried to duplicate mill glaze in the laboratory. The tests he conducted included planing lumber with dull blades at high input speeds.

Despite their best efforts the staff at the Forest Products Lab were unable to create a "glazed" surface. This result does not necessarily mean that mill glaze is not real or cannot happen...it merely means that in this instance the researchers from the USDA were unable to duplicate the effect of "mill glaze" in the laboratory.

Although no further research on "mill glaze" effect has been conducted at the USDA Forest Products Laboratory, they 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.

Protect your investment and do it right the first time! Remember, installing and protecting your new deck correctly from the outset will ensure enjoyment for years to come while preventing the need for premature deck refinishing, repair or replacement.

Please join us in our next blog as we continue to examine some causes of failures in wood coatings and offer suggestions on how to correctly install your deck and apply additional coatings/finish that will protect your investment and minimize the problems of cracking, raised grain, and mildew growth.

By Bill Christou, 10/16/17

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