FInishing room for Polyurethane finish

Polyurethane, or PU, is one of the most popular finishing materials in the wood furniture industry. It is widely used for furniture products that require a strong, durable, and high-quality finish layer. Items such as dining tables, kitchen cabinets, bar tables, and hotel furniture commonly use PU clear coatings. PU finishes are also preferred for furniture that requires a thick and luxurious film layer, such as Italian or Japanese-style designs.

Many furniture manufacturers in Asia favor PU finishes because they produce thicker, glossier, and stronger coatings compared to other finishing materials.

PU Finishing Process

The finishing process using PU is generally similar to other finishing systems. The basic principle of wood finishing includes applying stains and glazes to create color, followed by clear coatings (sealer and topcoat) to build the protective film layer. In a PU system, both the sealer and topcoat use polyurethane materials.

However, PU finishing is more challenging than NC (nitrocellulose) systems due to its longer drying time. This extended drying period requires more patience, time, and careful handling during the finishing process.

Once cured, PU coatings cannot be washed off or softened with thinner, which means any mistakes must be corrected before the film dries. If the PU layer hardens, repairing or stripping it off becomes difficult and labor-intensive.

Because of its long drying time, a PU finishing setup requires a larger and cleaner workspace. PU coatings are sensitive to both dust and humidity. Therefore, the finishing room for PU should be cleaner and more spacious than that used for NC finishes. For the same production capacity, a PU finishing area usually needs about twice the space of an NC finishing area. While NC, acrylic, shellac, varnish, or even water-based finishes can be applied in simple rooms—such as a garage or open yard—PU finishing requires a fully enclosed, dust-free environment.

Spraying and Drying Room for PU Topcoat

PU clear coatings require significantly longer drying times compared to NC or other finishing materials. Outdoor PU coatings take even longer to dry, meaning more space is needed to accommodate drying furniture.

PU is extremely sensitive to dust during drying. If dust falls on the wet surface, it will adhere to the film, resulting in a rough and dirty finish. This problem is even more visible on white or black finishes. For this reason, the drying room for PU topcoats must be designed to be completely dust-free.

One simple but effective method to reduce dust is by keeping the floor wet. A damp floor traps dust, preventing it from circulating in the air. Many furniture manufacturers successfully use this method to maintain cleaner environments.

Another option is to install air conditioning in the finishing room. This not only helps control dust but also maintains low humidity levels, reducing potential problems related to moisture during drying.

In the spraying room, air is continuously drawn out through the spray booth. To replace the air being removed, clean filtered air must flow into the room. The filters used for incoming air should be of the same quality as those in the spray booth to ensure the air remains free of dust and contaminants.

Room for Spraying and Sanding PU Sealer

Because PU coatings require longer drying times, the room used for the PU sealer should also be spacious enough to hold all furniture pieces according to production needs. Although the sealer room does not need to be as clean as the topcoat room, a cleaner environment still improves finishing quality.

PU sealers require thorough sanding, even more precise than with NC finishes. Since PU hardens through chemical reactions, each film layer becomes permanent and does not melt when new layers are applied. Therefore, any sanding marks or scratches left on the sealer will appear in the topcoat layer.

To prevent these defects, PU sealers should be sanded carefully using fine sandpaper—typically between 400 and 600 grit (U.S. standard). In some cases, wet sanding with even finer sandpaper may be needed to achieve an ultra-smooth surface. Proper sanding also prevents problems such as orange peel texture, ensuring a flawless final finish.

Storage of PU Materials

PU coatings cure through chemical reactions and have a shorter shelf life than single-component finishing materials. While NC materials can often last more than a year, PU materials should not be used more than six months after opening.

The PU hardener (isocyanate) is particularly sensitive to moisture in the air. When exposed to humid air, it reacts with water vapor and hardens, making it unusable. To prevent this, always ensure PU hardener containers are tightly sealed immediately after use.

It is safer to store hardeners in several smaller containers rather than one large can. A small leak can cause the entire hardener supply to harden within just a few days. For optimal storage, keep PU hardeners in a climate-controlled, dry room to prolong their shelf life and maintain consistent performance.