I built a wood fence this past summer, and the stain I used made a big deal about mill glaze. This was a new term for me, so I had to look it up.

It seems to be something that stain manufacturers and many online sources talk about, but theres little evidence based research about it.

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.

…

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.

— Mark Knaebe, USDA Forest Service, Mill Glaze: Myth or Reality?