What is “mill glaze”? It depends on who you ask. Mill glaze is a generally accepted industry term referring to a condition in which smooth lumber has difficulty accepting stain or holding paint. We've actually seen products crop up over the years with those two words in their name!
On the one hand, the condition is real—and should be properly addressed. On the other hand, we believe in sharing factual information so our readers are fully informed. With the help of Paul Mackie (known as “Mr. Cedar”) from the Western Red Cedar Lumber Association, we’re tackling the myth of mill glaze in the paragraphs below.
How the Myth Began
Mill glaze is an urban legend promulgated by paint companies that sold a lot of product in the northeastern part of the United States. Over the years, their salespeople told builders to install the clapboard (1/2X6 clear, smooth face, bevel siding) raw, let it weather for six months to a year, and then apply the primer and paint. This method usually resulted in the coating system not sticking to the siding. When adhesion failure occurred, those same salespeople claimed that the coating didn’t adhere because there was mill glaze on the surface of the wood.
Mill glaze is what supposedly happens to the wood when it goes through the planer at the sawmill. The resin in the wood is reported to liquify by the heat generated from the spinning planer knives, and is left as a glaze of sorts on the surface of the wood. This reduces the wood’s ability to hold primer and paint.
But there are a couple of problems with this theory.
The Real Story
First, Western red cedar contains no pitch or resin to be liquefied by the planing process. Second, the Forest Products Laboratory at the University of Wisconsin (operated for the USDA Forest Service) has attempted to replicate mill glaze on a number of wood species (including those which naturally contain pitch and/or resin), and these attempts have not been successful.
Secondly, the accumulation of dirt and moisture (as well as UV degradation) occur when wood is allowed to weather for six months to a year. This in turn loosens the cells on the wood’s surface, and reduces adhesion. After only 12 weeks of exposure to sunlight, the wood has lost 50% of its ability to hold primer and paint.
Other Factors Affecting Adhesion
Some scenarios at the sawmill can negatively impact stain penetration or paint adhesion. Planer crush occurs when planer knives have dulled, resulting in porous surface cells on vertical-grain wood folding over slightly. Folded cells reduce the openings which otherwise adhere to stain and paint.
Another likely culprit is smooth, flat-grain lumber. In flat-grain lumber, porous cells lay flatter than in vertical-grain lumber. This can impact adhesion of film coatings and penetration of stain products. Most of the tight-knot Western red cedar we sell for decking these days falls into this category, making it even more important that we are all on the same page about how to address it.
Installing lumber with a textured face avoids both of these potential challenges, but that is an unlikely solution when building a wood deck.
How to Address Mill Glaze
In Part 1 of this series, we covered more on the condition referred to as mill glaze including how to recognize it, and other tips for prepping your deck lumber properly.
For more on the myth of mill glaze, consider the findings of the Forest Products Laboratory produced on behalf of the USDA Forest Service.
If you have a question for Paul or would like more information about Western red cedar lumber—visit their website. You can also check out our Dunn Lumber podcast episode which featured Paul in a discussion about cedar.
We hope this series has been informative and can serve as a resource for you well into the future—whether you're staining or restaining your deck.