Blushing problem in the furniture finishing

Blushing is one of the common finishing problems found in the finishing process. Blushing is the emergence of a thin haze layer on the finishing film layer. Blushing can be shown as a thin layer of fog, but in some severe cases, blushing may seem like a rainbow in the coating film. Blushing is caused by some moisture that is trapped in the coating film during the drying process. Water vapor in the coating makes the haze look like a rainbow in the film layer. Blushing can happen at the sealer and top coat. But the blushing in the sealer layer won't be a problem since it can be eliminated by doing sanding and recoating with the good clear coat. The blushing will become a problem if it is found in the last coat since it will affect the finishing look.

Blushing happens when we apply clear coating in the very humid environment. Water vapor in the air will come into the coating and be trapped inside the film layer. Some factors that cause blushing problems are the quality of the paint, too humid drying room, and too thick coating layer.

blushing at the wood finish 

How to prevent the blushing problem

• Use the good-quality finishing materials

Use high-quality materials. Purchase materials from trusted suppliers. Thoroughly inspect each material you use to ensure it's in good condition before application. If necessary, conduct trials and tests to ensure the finishing materials you'll use still function properly.

• Use good thinners

Thinner is added to reduce the viscosity of the material so it can be applied easily. Thinner is a mixture of solvents: the active solvent that works to dissolve the finishing material and the inactive solvent that is added to reduce the price. To see more about thinner, you can view it in my previous article: thinner for wood finishing. The quality of thinner will highly affect the finishing quality, so make sure you use the high-quality thinner. This means the thinner should contain enough active solvent and be compatible with the materials. The good thinner should have enough active solvent and cannot contain too much inactive solvent. Too much inactive solvent in the thinner will disrupt the evaporation and drying process and could bring the blushing problem.

• Follow the drying time of each step

Each layer of finish must be completely dry before being applied with a new layer. An undried finish still contains solvent, and when material is added to it, the solvent will have difficulty escaping and could potentially become trapped within the finish, causing blushing. Therefore, follow each finishing process properly, and make sure you follow the drying time to ensure there are no problems with the resulting finishing layer.

• Avoid applying the clear coating too thick. 

The applications of the clear coating that are too thick will make the evaporation process become difficult and take a long time; this will invite the occurrence of blushing. Coating application should be done in a normal thickness. One layer of clear coating, or a maximum of 2 layers, followed by adequate drying and sanding, is safer and will produce a better surface finish than applying many wet coats at one time.

•  Equip the finishing room with an oven.

Oven is a facility to control the air temperature and humidity. When the air temperature is cold and humidity is high, then the evaporation of the solvent will be slow and difficult. Water vapor in the air will easily enter and be trapped in the paint layer and cause blushing. The oven in the finishing room will help to control the humidity and temperature of the drying room. prevent these problems. With the oven you can control the temperature and humidity in the finishing room, and by then you can control the drying process of your finishing layer. 

• Add lacquer retarder in the mix of the clear coating. 

Retarder is a solvent that is used to slow the drying process. Retarder usually is added in the mix. The addition of retarder will make the drying process of the top coat run slowly and will provide more time to let the water vapor in the coating layer get out and be released from the layers of film formed. Many blushing cases can be solved with the adding of retarder to the coating mixture, but it must be done carefully. Avoid too much retarder, since too much retarder will make the drying process take longer and slow down the entire production process. The addition of retarders can also affect the gloss of the coating films produced.

How to repair a finish affected by blushing.

Blushing that appears on the finishing layer must be repaired to produce a high-quality finish. There are several ways to repair blushing problems, depending on the type of finish and the degree of blushing present.

• Spraying a slow-drying thinner onto the surface

The thin blushing in the soluble coatings, such as NC, vinyl, acrylic, or single-component materials, can be removed by wetting the finished surface with a slow-drying thinner. A mixture of thinner and retarder can be sprayed onto the blushing surface to rewet the coating, hopefully allowing any moisture trapped within the coating to escape as the paint dries.

• Rubbing the surface with steel wool

For thin, superficial blushing, the blush can be removed by rubbing with steel wool or sandpaper. This rubbing process must be done carefully, as it will leave scratches on the surface. Ensure any scratches left on the surface do not affect the appearance. If the scratches are unacceptable, repainting will be necessary.

This method is also only suitable for thermoplastic coatings such as NC, vinyl, acrylic, and other one-component coatings.

• Recoating and refinishing

For the severe blushing, where the blush is thick and cannot be removed with the previous methods, the blushing can only be resolved by refinishing. Sand the affected area until it's completely gone, and then reapply a clear coat until you achieve a smooth, clean, even surface. If necessary, you may need to wash and remove the entire finish layer and refinish the wood. You need to sand the blushing part until it is totally removed and clean and then recoat with another layer of top coat or sealer. If necessary, you may have to wash and remove the entire finish layer and refinish. 

This last way is usually needed when the blushing problems happen at the thermosetting coating material, such as PU, AC, and the other two-component coating types.