Greg Williams has been involved in the furniture industry since 1971, and has been intimately involved in the larger wood finishing world for more than 35 years. He is retired from the RPM Finishes Group where he formerly served as Senior Instructor and Manager of Education for Mohawk Finishing Products and H. Behlen Brothers. He has taught finishing and touch- up and repair to students around the world, and is currently an independent consultant to individuals, companies, and organizations. He can be contacted firstname.lastname@example.org.
What Went Wrong? Predicting Performance
When I first began finishing, on an old gunstock, I read an article that described a simple, time tested procedure and followed it. It worked fine. The first sprayed finish I did was on an army helmet liner, and I was instructed very specifically how to do it. It worked fine. Later, when I worked in a furniture warehouse, I was further educated in procedure by an experienced finisher. I learned how to do specific jobs, but I had little understanding of “why” it was done that way, just that “this way works”.
When I eventually began selling finishing products I was faced with trying to figure out why some things didn’t work, as my customers would challenge me with scenarios of failure. 123
Here’s one. See if you can figure out what happened.
A custom shop fabricated a large, expensive conference table which was made ready for finish just before the Christmas holiday.
The finisher was in the habit of using a high quality lacquer from one manufacturer we’ll call “A” and a good, but less expensive thinner from “B” with acceptable results. Generally the finish work was completed within three days. The first coat of lacquer was applied on December 24 and the last coat was applied January 2. On June 4, the top of the conference table had to be redone. Minor surface abrasion such as that from coffee cups, pens, and even fingernails were leaving highly visible white lines which didn’t even penetrate the topcoat. More vigorous abrasion removed chips of that final coat.
What went wrong?
Thinner B, while perfectly suited to lacquer B, had much less active solvent than required by lacquer A. It reduced viscosity adequately, sprayed and flowed out well, but had less solvent “bite”, or the ability to “burn-in”, than thinner A.
This finisher’s normal procedure was to apply several thin coats in a short time so that previous coats were somewhat open, not fully dried, when subsequent coats were applied. Consequently he normally achieved good intercoat adhesion due to solvent bonding between coats. When the lacquer was allowed to dry over the holidays the last coat lacked the solvent power to adequately resolvate, or liquefy the previous coat allowing the establishment of a solvent bond.
The transportation, labor, and material required to refinish the tabletop cost the finisher more that he had saved all year by buying the cheaper thinner. It isn’t cost effective for a finisher to try to second-guess the manufacturer by economizing on thinner.
Here’s another costly situation. A finisher in a large retail furniture warehouse has a workspace in the northwest corner of the warehouse , enclosed on two sides by 2x4 framing and drywall, with a dropped ceiling at 10 feet.
Above and around this area is the marginally heated warehouse. The heat is turned down to 50 degrees on Saturday night, and since the warehouse is closed on Sunday and Monday, is not turned up until Tuesday morning. The drywall on two sides of the workspace is unpainted, and uninsulated. The other two walls are of unpainted, unsealed cinderblock. The floor is concrete, from which the sealer has long since worn away.
The finisher was charged with doing a burn-in repair on the top of the expensive dining room table. This table was new, had been damaged in an attempted delivery, and now was to be repaired and returned to the unhappy customer. The original salesperson had “promised” the customer that the table would be redelivered within a week. (without consulting the repairman).
The finisher did, indeed, do a masterful job on the repair, but determined that to satisfy this customer he would have to recoat the entire top. The repair was done on Tuesday morning. The air in the repair area was chilly and damp, as it had rained over the weekend and was still drizzly outside. The finisher did have a small space heater in his area, which made it more comfortable and somewhat dryer.
Anticipating the possibility of blushing, he added a small amount of retarder to his lacquer before spraying the top. Unfortunately, there was a slight blush appearing as the lacquer dried. He allowed that first coat to tack off, then resprayed the top, this time with a mixture of about 30 percent lacquer, and 60 percent thinner and 10 percent retarder. Needless to say, the lacquer, thinner, and retarder were from three different manufacturers.
The blush disappeared, and the lacquer leveled well. As it dried, the finisher noticed that the burn-in was now somewhat raised above the surface, as if it had not been properly leveled. He left the shop for the day, expecting to cut down and rub out the top the next morning, or perhaps the following day (Thursday) in time for a Friday delivery. During the evening, the heat was again reduced in the warehouse, and the relative humidity increased as the temperature dropped.
On Wednesday the lacquer appeared too soft to cut down, and the burn-in was still too high, so the finisher decided to respray the top, this time without adding any retarder, in the hope that the next coat would allow him to “level” the burn-in. While that coat was tacking off, it looked good and the burn-in was not as evident as before, so the finisher put the table away to dry and occupied himself with other projects.
On Thursday morning, he almost panicked and quickly called the supplier of the burn-in sticks to ask for help, and to complain about the defective sticks, as the raised burn-in was the most obvious “defect” in the finish at this point.
What Went Wrong.
The retailer was maintaining horrible conditions for spraying lacquer by turning the heat down so far at night, and by constructing the shop area with 4 unsealed walls, two of drywall and two of cinderblock, over an unsealed cement floor. These surfaces hold moisture for days, keeping the humidity high even when the heat is turned up.
To combat the tendency of the lacquer to blush, the finisher added retarder, a slow evaporating solvent designed to hold the lacquer open longer than normal in order to allow the moisture to escape the drying finish. Because he was using thinner, lacquer and retarder from different manufacturers, the technician was not able to easily determine if the “mix” of those products was the best, or even appropriate, for the task. In this case the lacquer thinner was already of a “slower” or “winter” version, that is, it already contained a higher percentage of slower evaporating solvents designed to balance a lacquer which contained less of those solvents. The lacquer, however, did contain a higher percentage of the slower solvents, and was reputed to be quite “blush resistant” under normal conditions.
In the situation we are describing, the lacquer was cold. The thinner was cold. The air in the compressor was cold. The temperature by itself would slow the release of the solvents from the lacquer, and from the burn-in, which was saturated with slow, cold solvents. Adding retarder in this case, while it did help prevent blushing, only exacerbated an already bad situation.
The resins in the burn-in hold solvents much longer than the lacquer, which is one reason that the suppliers of the burn-in material recommend light coats of lacquer over a burn-in. The several coats of retarded, cold lacquer were then covered with another coat, without retarder, which dried faster, almost locking in the unevaporated solvents and diluents in the previous coats, and in the burn-in. Those trapped solvents kept the burn-in swollen, or “raised” above the surface of the wood, showing a “bump” on the surface.
There is a happy ending to this situation though. The customer was called and the problem explained in terms of bad weather slowing down the finish drying. The upside of the repair, for her, was that she would have more coats of a very durable topcoat than the original, or a replacement table, would have, increasing its value and utility. She agreed to wait for another week. In 3 days the “swelling” of the burn-in was gone, as the additional dry time allowed the solvents to escape both the burn-in and the lacquer. On the 5th day the finisher cut down the top with 400A Wet or dry sandpaper, followed by 600, then 1200, sanding with a block and water soluble lubricant. He then used a buffer and fine polishing compound to bring back the original sheen. 137
Using a different thinner than recommended, even a comparably priced, high quality thinner, can cause the lacquer to behave in a different manner than expected. It could lift or dry slowly, or cause stain to bleed through the sealer. Quality reducers are formulated carefully by the manufacturer to complement the solvents and diluents already in that company’s lacquer, sometimes even a specific formulation of that lacquer.
Research and testing under varying conditions have been performed so manufacturers can reliably predict the performance of their finishing materials. While other thinners may perform satisfactorily under some conditions, especially where a fine finish isn’t a high priority, small variances in application conditions can cause unexpected and expensive results. The slowest solvent in a reducer should have good solvency for the resin system. It shouldn’t be a diluent type solvent that could become trapped under the dry film.
Read the directions. Use the coating manufacturer’s recommended additives. Don’t experiment on the customer’s furniture.