Polyurethane is a type of protective coating that tends to form a solid, impervious layer similar to that of a plastic film when it is applied on a wooden surface. This coating generally comprises a chemically curing resin system that offers an advantage when compared to the standard lacquers or varnishes. It is extremely flexible and hence, works well even when the surfaces are curved. Polyurethane coating gives wooden surfaces an excellent resistance to scratches, abrasion, spills, rough usage, etc. Additionally, it provides high resistance to chemicals such as acids or alkalis and boiling water and even protects from strong UV rays. If you are wondering what PU coating on wood is, here is everything that you must know.

Types of Polyurethane Coatings

Polyurethane is typically classified as a water-based and solvent-based coating.

The water-based polyurethane offers low VOC level, emits less odors, and dries at a speed that depends on the outdoor temperature, to form a transparent and clear coat. These coatings though, provide slightly inferior properties compared to solvent based coatings and are more expensive than solvent based coatings.

The solvent-based polyurethane coatings, on the other hand, have a much stronger smell compared to the water-based polyurethane coating due to the presence of solvents. It may appear stronger in odor depending upon the sensitivity of an individual.

Areas of Application 

  1. The polyurethane coatings are recommended to be used for wooden furniture or surfaces that are subjected to rough usages, like windows, and wooden doors.
  2. The PU coated wooden furniture demonstrates better performance and resistance to moisture compared to traditional wood coating products.
  3. Another such application of polyurethane is the wood floors wherein it could provide high level of abrasion resistance and an excellent aesthetic finish.

This finish offers excellent exterior durability and UV resistance, however, some of the variant have limited exterior durability.

Types of Finishes

Polyurethane provides an extremely high-end luxurious look to the wooden surfaces. It is available in different kinds of finishes such as satin, high gloss and semi-gloss. The level of glossiness does not affect the durability of the coating.

When and How to Apply Polyurethane Coating on Wood

When one applies polyurethane coatings on wooden surfaces, one gets a special level of finish that is far superior compared to that of single pack varnish or lacquer. It also offers advantage w.r.t. surface hardness, mechanical resistance and chemical resistance.

The two-component polyurethane coatings are more resistant to different atmospheric agents, provide a greater surface hardness, and guarantee you better adhesion on the substrate. Due to these characteristics, it is advantageous to use a polyurethane coating for wood.

Ligna, a brand of premium wood coatings by Grand Polycoats, offer a complete solution not only protecting the wood, but also adding aesthetic appeal. One of the products offered by Grand Polycoats is LignaCoat 181, a high gloss, premium 2K clear coat that provides excellent durability from UV, dust and water. This product in also offered in a dead matt finish to suit the latest trends.

The polyurethane coatings are generally quite versatile and are applied by various application methods like:

  1. Spray gun
  2. Airless
  3. Curtain coating
  4. Automatic and electrostatic systems

Irrespective of the type of products you choose or where they are applied, for accomplishing a good result, you must consider these factors:

  1. Surface area to application method and paint that would be used.
  2. For two-component polyurethane coating, follow recommendation as per the manufacturer, mix base and hardener as mentioned and use recommended thinner and apply as per application guidelines.
  3. Polyurethane coatings cannot be applied on damp surface or when the relative humidity is beyond 60%.
  4. Mineral spirits is generally not used for polyurethane coating.
  5. In the case of touch up or minor correction work on smaller surfaces that need attention to detail, use a smaller classic brush.

Application Techniques

Following are the steps for applying polyurethane coating on wood.

  1. The very first step is to prepare wooden surface by sanding with appropriate emery paper till smooth surface is achieved. Apply recommended sealer coat to make it smooth, even and impervious. Ensure the surroundings and working environment is clean and dust-free to prevent any dust or dirt from sticking onto the surface when it is wet.
  2. Ensure no open electrical cables, wires, welding work, open flame, welding or smoking takes place near to the paint application area.
  3. It is crucial to keep the area very well ventilated. To do so, switch on the fans/exhaust fans so that any sorts of fumes emitted by polyurethane would be instantly replaced with fresh air.
  4. You can also use a respirator to avoid inhalation of toxic fumes. As a precautionary measure, keep older people and children away from the site/area.
  5. Make sure area of application is free of moisture, as high moisture content will affect the curing of PU and its quality.
  6. Open base container and slowly stir the content to homogeneity, do not shake hardener, slowly add it to base in recommended ratio. Close the lid of hardener container immediately after use. You need to read all the instructions as mentioned by the manufacturer. Use recommended thinner only.
  7. Mix the required quantity of paint, sufficient enough for the application. As product has a limited pot life of 4-6 hours, excess quantity will gel and become unusable.
  8. Then apply the first coat of polyurethane on your wooden surface with the help of a brush, spray or a roller. When you apply it on the vertical surfaces, there are chances that the coating might drip. In this case, you must sand it as soon as it dries. Allow the polyurethane coat to dry completely before you proceed.
  9. Re-sand the coating before you apply the next coat.
  10. Repeat these steps until you are satisfied with your desired level of finish. Remember that all indoor furniture would need at least two coats of polyurethane coatings. The wooden surfaces would require three coats to get optimum protection.
  11. The drying time between each coat would generally be two hours at 30 degrees, however it may vary depending upon environmental condition. It is advised that you wait for a minimum of 24 hours after the application of the final coat before you use the wooden surfaces or furniture since the polyurethane coating might be sticky.

Acknowledgement

The author is thankful to the entire management team of Grand Polycoats for giving us an outstanding opportunity and encouragement for conducting the required experiments. The author is also grateful to the Research and Development team at Grand Polycoats who greatly contributed towards data generation and helped to conduct experiments smoothly.

Grand Polycoats is the leading manufacturer of industrial paints having specialization in providing high-performance paints and protective coatings. They cater to the requirements of different sectors like automotive, construction, power and electrical, defense, and process industry through their extensive product portfolio.