Author – Phil Stevenson, AWFI Founder
As the weather heats up during the summer, finishers should expect finishing problems to escalate. As a general rule, ambient temperatures over 90 degrees with accompanying humidity over 70% can cause problems to occur in finishing process. This is especially true when both temperature and humidity are excessive. Every finisher needs to be prepared for these summer time conditions. Here are some common finishing problems and how to adjust to suit these inclement weather conditions.
White Wood Sanding Issues
High humidity can cause several problems in the raw wood. Moisture from humidity can raise the wood fiber and cause excessive sealer sanding and removal of stain on sharp edges during the sealer sanding process. Stain color consistency can also be problematic with excessive board surfaced moisture.
The first line of defense for working under these conditions is to cover raw wood product with black plastic before final sanding. Black plastic will prevent the humidity from attacking the wood surface. It is not recommended to use clear plastic, which will allow the moisture to condense under the plastic, allowing moisture content to increase.
It is always best practice to perform the final white wood sanding step no longer than 12 hours prior to finishing. This will minimize the moisture from re-raising the wood fiber and causing the stain to penetrate into the wood inconsistently.
Sanding swirl marks (fish hooks) that are visible after finishing can be more problematic when high humidity is present. Swirl marks are small scratches left in the wood surface from the random orbit sanding process and are considered to be acceptable when they are not visible after finishing. However, under high humidity, the size of the scratch marks will become bigger due to the moisture swelling the marks. After finishing, these swirl marks will become visible and will be considered a defect. To eliminate or minimize the problem, be sure to check the following:
- Make sure the sand paper is not wrinkled or damaged and the back up pad is defect free.
- Change the sand paper more frequently to insure that the sanding abrasive is providing a clean cut (of the wood fiber).
- Use the best type and grit size recommended by your abrasive supplier.
- Make sure the sander is working properly; for example, be sure to keep the sander well lubricated with the correct air pressure to insure optimum performance.
- Keep the sanding surface clean and free from dust building up under the abrasive paper.
- Use correct sanding techniques as recommended by your sanding equipment supplier.
When swirl marks become visible as a defect, you must re-sand to bare wood and refinish.
Finish Coating Problems
It is always recommended that you try to schedule finish application during the cooler hours of the day. Heat will increase the speed of the solvent’s evaporation rate from a liquid to a vapor. Most finishes are formulated to work best in ambient temperatures from 70 to 80 degrees Fahrenheit. Let’s take a look at the most common problems that you may encounter.
1. Stains
Wiping stains may dry too quickly during high temperatures to allow for uniform application. To retard the flash-off rate of wiping stains, add 2-4% of Aromatic 150. Mineral spirits can also be used to retard the stain by using up to 3%.
On large, open pored wood such as oak, high temperatures may cause the stain to bleed out of the open pore. The solvents are evaporating very quickly and are pulling the pigment out of the bottom of the pore onto the surface of the wood. This defect will look like dark specks directly around the open pore areas of the wood. Use 4% of Solvesso or Aromatic 150 to help eliminate this problem. Additionally, keep the product out of direct sunlight to prevent the stain from flashing off too quickly.
2. Sealers and Top Coat Problems
Lacquers are very susceptible to blushing during the spray application of the material under hot and humid conditions. This blush defect will look like a whitish cloud-like haze. Millions of small micro-bubbles of moisture become trapped in the finish to cause this defect. To minimize the problem, use thinner coats of lacquer with ample dry time between coats. If the blush continues to persist, use 2-4% butyl cellusolve to retard the evaporation of the lacquer. If the lacquer needs to be reduced and retarded, use methyl amyl ketone at 5-15% of the mixture.
To remove blush from the finish without having to refinish, spray a slow evaporating lacquer thinner over the blushed area as soon as it appears. This will re-open the top surface of the film of lacquer and allow the little bubbles of moisture to escape.
3. Conversion Varnish and Pigmented Catalyzed Finishes
Temperatures exceeding 90 degrees F and 80% humidity can cause varnishes to trap solvents in the finish. When “solvent entrapment” occurs, the top surface of the film is drying too fast to allow the solvent to rise to the top and escape out of the coating material. The solvent will form vapor bubbles, causing visible defects. When this occurs, the only way to remove the defect is to sand down through the finish to remove the bubbles and then to recoat. To prevent the solvent trapping from re-occurring, use 2-4% butyl cellusolve retarder in the coating mixture. Xylene works very effectively to reduce viscosity of the coating. This will allow lighter coats to be applied to minimize the problem. As a general rule, no more than 5% Xylene is needed to achieve good flow out.
Excessive heat can cause open pored wood to blister when finished. With this condition, the solvent does not escape out of the deep pores before the finish starts to skim over on top. The solvent vapor tries to escape and will push the finish up into a blister defect. Use 4% butyl cellusolve in the finish mixture and keep the finished product away from heat sources and direct sunlight. To remove the blister defects, lightly sand with 320 grit after the coating has dried and apply an additional top coat.
Hopefully, with these tips, you can keep production rolling during the long, hot summer months.