Understanding Why Burnished Wood Fails to Hold Finish
Oxidation of heated wood surfaces explains why heating up wood during machining or sanding can make the surface resistant to adhesion of various finishes or adhesives. October 19, 2014
Question
We had some paint adhesion problems on a kitchen we built two years ago. Our finish supplier sent a door to their lad. Here is the result of their tests.
“Mill glaze is the smooth glossy-like surface that develops on newly milled lumber during the production process. Heat from saws and shapers will draw the natural sugar like minerals to the surface and as they cool will harden creating a sealed surface not allowing product to penetrate and bite properly causing premature pealing and coating failure”
Forum Responses
(Finishing Forum)
From contributor H:
Did you sand the doors prior to finish? It seems to me that would render their statement about newly milled lumber and generated heat moot. Sharp planers and shapers with proper feed speeds don't usually cause this issue. If they were dull then the ball could be back in your court. In any case I wouldn't expect the lab to own up to anything except "it is your fault"? I'd drop their finish coating like a hot potato.
Some years back I had doors crazing in the finish about five years after installation. The finish people said it wasn't their fault which left me holding the bag. I suspect the finish was double catalyzed by them (post cat lacquer) but don't know for sure. Didn't help my issue and was never sure the exact cause. I changed coating brands and now watch every container being catalyzed and the amount used. If you are sure you did everything correctly you might fix what you have and change brands. It isn't always the craftsman who screws it up but we are the last person in line so get the blame.
From the original questioner:
We did sand the doors. My opinion is that maybe we oversanded/burnished the doors. We remade the doors and it is behind us but I thought everyone would find the story interesting. We have since changed brands.
From Gene Wengert, forum technical advisor:
From a wood technology standpoint, the explanation of why wood glazes over is not correct. However, heat can indeed cause a chemical change at the wood surface, whether it is heat from a knife, dull sandpaper, infrared lamp, etc. This change starts around 200 F. If it exceeds 300 F or so, then the wood starts to char and you see a dark mark. This charring is actually very slow burning. From a technical point, wood has three basic chemicals - cellulose, hemicellulose and lignin that make the wood cell. Within the cell itself we can have all sorts of additional chemicals that give wood its color, aroma, decay resistance, etc. The heat causes these three basic chemicals to oxidize quickly, making them chemically unattractive to a finish, adhesive and even water. In fact, we can put a drop of water on a surface and see if the water is absorbed or sits like a drop on a newly waxed hood of a car. If the drop sits the surface is called inactive. This drop test is especially useful when gluing but can be useful prior to finishing as well.
From Contributor W:
Heat and compression of the sanded surfaces commonly causes this issue. Too often the wide belt sander is used incorrectly, closing off the surface by work hardening and compressing it.
