Wood Finishing 101: The Chemistry in Coating Wood Furniture
By Matt D'Anca | Posted: 07/11/2014 9:00AM
In the woodworking industry, there is a tendency to place manual finishing in the realm of art, completely ignoring the scientific aspects in any way. This generally works out if you are meticulous, use systems exactly as the manufacturer recommends, and don't cross contaminate your products, but the reality is that more finishing problems are the result of microscopic interactions than flawed 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 always liked chemistry and I was fortunate to have very good chemistry teachers.
When I've been asked to assess finishing problems in the past, my first question is, ”What gun did you use? Can I see it?” Nine times out of 10, the gun is filthy, and has been used to spray finishes in completely different families. When we talk about families in the finishing world, we are talking primarily about solvent families.
Which brings us to our first principal of finishing chemistry: ”Like dissolves like.” Dissolution (when one state of matter is dissolved in another: e.g. Saltwater). But here's the rub, not all finishes are solutions. Many modern finishes are suspensions. Suspensions are like milk. Milk has lots of fat molecules suspended in water. Fats don't dissolve in water, and given time, the two will separate.
Blah, blah, blah...boring! Right? Well, if you aren't bored yet, this next part will probably cause you to spit at your computer screen: Why does it matter if the finish I'm using is a Solution or a Suspension? well, in many ways it doesn't. At least not if you get the expected result.
But not all things work the same way backward as they did forward. Take the example of a tabletop I was asked to refinish last year. I took it as a subcontract job from another woodworker, and I should have known better, but I kinda owed the guy a favor.
The veneer that was initially applied started bubling, and so my colleague reapplied veneer, and sent it to me. He included the finish he used...and shortly after I applied it, the problem became evident. While the original problem was bubbling, I experienced blushing in spots on the surface. As I showed him the problem, he kept telling me that the problem was that I wasn't putting enough finish on in each pass. But I was laying the finish on 10 mils wet thickness, and the coat was even. I asked him how he had adhered the veneer to the substrate, and the answer explained the rest: Contact cement.
About the Author
Matt D'AncaMatt D’Anca, president of D'Anca Design, Inc., a custom woodworking and metal fabrication firm in Cicero, IL: "Using our own designs, based on customer's needs and specifications, we create top quality custom woodwork, metalwork, and mixed media pieces, including heirloom quality furniture, fine home interiors, and small batch manufacturing of short-run (under 100 units) of product for our clients." - M.D. He holds a B.S. in Environmental Chemistry.