Wooden doors production involves many steps from wood selection, dewatering, veneer plaster, Polish and painting etc. The production process determines door quality directly. Especially painting techniques, it will bring direct impact on finall products. Good painting not only make wood veins clearly visible, it also make wood doors water proof for longer life cycle.

Types of paint on the wood market are divided into phenolic paint, alkyd paint, polyurethane paints, Nitro paints, polyester and PU paint. Phenolic paints and alkyd paint are being dropped away due to poor adhesion. Nitro, polyester and PU are widely used.

Nitrocellulose lacquer is simple solution and mostly used for manual operation. This kind paint provides thin film thus is not ideal choice for high quality door production. Polyester paint film is relatively more thick, but its diluent containing radon in the volatile and film hardness is slightly weaker. PU paint is the best, it provides all advantages from polyester paint, while it’s excellent in anti moisture. Further more, the PU coating film hardness, durability, resistance to yellowing and environmental protection properties are far more better than other paints. The general painting work flow using polyester and PU must go through color, three base coats and two top coats procedures.

For wood door manufacturer, high quality wooden doors paint choice and strict production workflow is crucial for door quality. Wooden door can be coated by many different processes, the actual process can be vary upon customer requirements and standards for appropriate choice of coatings and coating line configuration. From the wood door surface requirements, we can divide them into following categories: fully closed and semi-open. From color view, it can be as transparency and solid color effects.

On the market, the majority of wood door products coating effect is fully enclosed and clear coloured effect, the choice of paint is mainly PU based.

We’ll introduce the main workflow and its subflows used by different companies

Case one – One base coat & one top coat

1, Process: Sanding → water sanding → sanding → transparent primer coating → sanding → colored top coating.

2, Product market: low-grade products.

3, Advantages: simple production and the lowest cost for painting, broken surface can be easily repaired by water base paints. Dark color painting be able to cover the substrate properly for unified color.

4, Weakness: Dark color painting makes it impossible to show natural texture of wood beauty, the whole painting effect is similar to MDF for solid color paint effects. Because of only one primer, surface hardness is bad and can easily cause wave like film. Water paint polishing is most time consuming and labor intensive, it produces a lot of dusts thus its harmful to operators’ health.

Case 2 – 2 base coat & one top coat

1, Process: Sanding → water sanding → sanding → transparent primer coating → sanding → transparent primer coating → sanding → colored top coating.

2, Product Market: middle grade.

3, Advantages: technology is relatively simple, low cost coating with better quality than case one.

4, Weaknesses: Product permeability is not very good, can not show up the natural texture of wood. Water paint polishing process is time consuming and it produces a lot of dusts.

5, Improvement suggests:

• Change water paint to “colored water paint”
• Change colored transparent surface painting to color repairing painting with slight sanding then spray clear top coat. In this way, surface physical properties will be greatly improved and the wood’s texture is more clearly maintained, there won’t be too much overall cost difference.

Case 3 – 3 primer & one top coat

1, Process: Sanding → water sanding → sanding → transparent primer coating → sanding → transparent primer coating → sanding → transparent primer coating → sanding → color repairing → slight polishing → top clear coat.

2, Product market: Medium to high grade.

3, Advantages: good permeability, can clearly show the natural texture of wood. no need of water paint sanding and reduced the man power for sanding.

4, Weakness: high costs, process is not simplified. Can not enhance the wood texture effect but just show as it is.

5, Improvement suggests:

• if the material color difference is big, the second primer can be used wet on wet operation
• or change the second primer coating to sanding/coating, not only cost savings but also result in better quality.

Case 4 – colored primer & repairing top coat

1, process: Sanding → protective primer spray → sanding → spray PU grace → spray transparent primer → sanding → spray transparent primer → sanding → spray transparent primer → sanding → color repairing → slight sanding → spray clear coat.

2, Product market: high grade.

3, Advantage: provides very good permeability, very strong 3D effect, even the wood texture is not clear itself, it can be cearly displayed through above process.

4, Weakness: high cost and process duration is long.

5, Improvement suggests:

•   if the wood texture is very clear and very beautiful, there is no need of coloring primer(such as Sapele veneer, direct transparent primer is enough for its flashing red effect, it is very beautiful; But if you use colored primer, such effect won’t be available)
•   Aftr coloring primer, lighter color repairing is suggested for better permeability.

Based above introduction and analysis, we see that every process technology has its advantages and disadvantages. We need to choose the right process by actual client requirement and production conditions. Or else it will be difficult to achieve the desired results.

In the wood industry, it’s a big course on how to control their costs, improve quality and increase production efficiency. Improve quality, does that mean to make costs high? No it isn’t.

For example, someone says, I want “coloring primer and coloring top coat repairing” effect but want to simplify the process for less cost. In fact, it is possible. Some factories already replaced the old water paints using new material. Such new material not only has very good filling properties and easy operation, but also be able to achieve the effect of coloring primer. The use of new materials can optimize processes, reduce costs and improve efficiency.

In early times, most people adopt the method of brushing over paint, later changed to spraying methods and greatly improved production efficiency and quality. But people may be not awared it has a variety of ways for coating, some factories use the shaving method for cost savings while improved efficiency, and final effect is much better. So the coating technologies can be developped and improved endlessly.

Wood door coating FAQ by Engineer

Q: My wood door surface is very smooth after painting, but after some time I found its not so bright and not so smooth on the surface, what is the reason?
A: Its mainly because of the film sag. It will cause surface irregularities and inequalities and declines light reflection.
Solution: apply well primer treatment, primer must be well polished and complete dry up. After that, the problem can be avoided.

Q: We managed to reduce paint costs, but the overall cost is still high, how can we reduce costs?
A: The cost control is about the entire coating line system thus just using cheaper paint will not solve the problem.
We can fix from the following three ways: first, select the most appropriate technology; Second, control the rework rate; Third, use experienced operators.