June 29, 2022

Wood Finishing 101: The Chemistry of Coating Wood Furniture

In the woodworking industry there is a tendency to place manual finishing within the realm of art, completely ignoring the scientific aspects in any way. It usually works if you’re meticulous, use the systems exactly as the manufacturer recommends, and don’t contaminate your products, but the reality is that more finishing issues are the result of microscopic interactions than faulty technique.

My experience is that most people shudder at the thought of chemistry. At best, when I start talking about it, I get a lot of blank stares. I have always loved chemistry and have been fortunate to have great chemistry teachers.

When asked to assess finishing issues in the past, my first question is, “What gun did you use?” Can I see it? ”Nine times out of 10 the gun is dirty and has been used to spray finishes in completely different families. When we talk about families in the finishing world, we are mostly talking about creditworthy families.

Which brings us to our first principle of finishing chemistry: “The genre dissolves like.” »Dissolution (when one state of matter is dissolved in another: for example salt water). But here’s the catch, not all finishes are solutions. Many modern finishes are pendant lights. Suspensions are like milk. Milk contains a lot of fat molecules suspended in water. Fats don’t dissolve in water, and over time the two will separate.

Blah, blah, blah … boring! To the right? Well, if you’re not bored yet, this next part will probably make you spit on your computer screen: why does it matter that the finish I’m using is either a solution or a suspension? well, in many ways it is not. At least not if you get the desired result.

But not all things backward and forward work the same. Take the example of a table top that I was asked to redo last year. I took it as a contract job from another carpenter, and I should have known better, but I kind of owed the guy a favor.

The veneer that was initially applied started to bubble up, so my colleague reapplied the veneer and sent it to me. He included the finish he used … and soon after he applied it the problem became obvious. As the original problem bubbled up, I got some redness in places on the surface. As I showed him the problem, he kept telling me that the problem was that I wasn’t finishing enough with each pass. But I applied the finish to 10 mils thick wet, and the coat was even. I asked him how he got the veneer to adhere to the substrate, and the answer explained the rest: contact cement.

While there may be differing opinions on the use of contact glue for solid wood veneer, one thing can be said for sure, the solvent in the finish was similar enough to the solvent in the contact glue to cause finishing defects. It made you blush when I applied it, but degraded the glue so much when it applied its [unmeasured] finish “enough” that the glue fails completely in places, causing bubbles.

There are ways around most problems in the woodworking shop, and knowing how to do it is always the result of knowledge. In the finishing room, this knowledge base includes an understanding of basic chemistry concepts. Stay tuned as I continue to share this knowledge which could be the final link in the quality of your finish!