Adhesion Problem in the Wood Finishing

Adhesion is the bond between 2 or more different materials. Finishing adhesion is the ability of a finishing layer (film) to adhere firmly to the surface of a substrate. Good adhesion of the finishing film is very important in the finishing. The good adhesion is needed to ensure that the finishing layer can provide the protection to its substrate underneath. Good adhesion means that the finishing layer does not easily peel, come off, or lift from its underneath surface. There is a standard test to observe the adhesion of the finishing layer; you can visit our previous article: adhesion test for wood finishing

Adhesion problem for finishing

Adhesion problems occur when the paint layer is easily peeled off from the surface. This peeled-off paint can occur on one layer only, for example, on the top layer only (top coat or sealer), or it can occur on the entire layer, meaning the whole finishing material from the stain is coming off all the way from the surface. This adhesion problem can be very severe when the finishing layer can be peeled off with just a little rubbing or light touch. To check the adhesion of the finishing layer, there are some adhesion tests that can be held to observe the adhesion quality of the finishing. The adhesion strength required may be different for each type of product. The products that prioritize usability, such as furniture for hotels, offices, and restaurants, will need higher adhesive strength than the products that are purposed to decorate and give aesthetic value. 

This adhesion problem, if it occurs, will be a serious problem. Products that are experienced with adhesion problems will appear as damaged products. No matter how good the product looks, it will be useless if the finish is peeled off. 

adhesion problem for bamboo finish

The causes of adhesion problems.

• Quality of the finishing materials

The quality of the finishing layer highly depends on the quality of the finishing materials used. The poor quality of finishing materials can be due to the production processes in the paint manufacturer or the incorrect handling in the finishing warehouse or finishing field. Finishing materials with good quality can be damaged due to several things, such as being expired, improper storage, improper mixing, inappropriate thinners, and improper application.

• Solvent trapped in the film layer

Most finishing materials contain thinner in the mixture, but all the solvent and thinner must be released in the drying process. The presence of solvent in the film layer will affect the strength of the finishing layer. Some adhesion problems are caused by solvents that are trapped in the finish layer. This is due to inadequate drying time, too thick a film layer, or improper application.

• Improper sanding

Good sanding is one of the factors that determines the quality of the finishing. For 1-component finishing materials such as NC, acrylic, vinyl, shellac, varnish, etc., the dry finishing layer will resolve and unite with the finishing material layered on it, so sanding will not highly affect the adhesion of the finishing layer. In this system, sanding is more intended to clean and level the surface. But for finishing materials that are not resolved, such as PU, melamine, UV coating, water-based coating, and other catalyzed coatings, the sanding process will greatly affect the adhesion strength of the finishing layer. Sanding not only functions to smooth the surface but also to form scratches on the film layer needed as a grip for the paint that is coated on.

• Substrate problems 

Substrates can be problematic when they are dirty or contaminated by oil, chemicals, or other dirt that interferes with the adhesion of the material to the surface. The improper sanding will also create problems in the finishing. Then the cleaning and proper sanding are needed for most of the surface as preparation before the finishing process.

The wood and veneer need to be cleaned and sanded properly as the first step of finishing. Do proper sanding according to the wood types and finishing process. To find more about sanding, you can read our previous article: Sanding for the wood preparation

Some engineered wood products, such as MDF, particleboard, HPL, decorative paper, etc., have a slick and thermosetting film layer on their surfaces. To produce a good bond, this surface must be well sanded to provide the scratches needed to produce a good bond.

Certain substrates, such as rattan skin, bamboo skin, have a very hard and solid surface and contain a kind of wax that will interfere with the adhesion of the finishing material. This substrate requires sanding and special materials to produce a good bond between the surface and the finishing layer above it.

Some other natural substrates, such as water hyacinth, pandan, banana leaves, and raffia, contain a layer of wax that has the potential to interfere with finishing adhesion and requires special materials to produce a finishing layer that can adhere strongly to its surface.

•  Wet wood or water problem

Water is naturally one of the biggest enemies of wood or other natural materials. Water in the wood will try to rise and come out and damage the finishing layer, which will look like an adhesion problem. 

Water will also invite fungi or bacteria that have the potential to damage the substrate and the finishing layer above it. This problem can also end with peeling paint and eventually become an adhesion problem. 

• Film layer that is not completely dry

The finishing layer will not reach its strength if it is not completely dry. A finishing layer that is dry with the evaporation of its thinner may not dry completely or may require a very long drying time due to improper thinner or being too thick film. Finishing material that is dry with a reaction may not dry completely due to low material quality, improper mixing, or an oven that does not match the specifications of the material.

Tips to avoid and anticipate adhesion problems

A finishing layer with adhesion problems almost cannot be repaired. The only way to repair it is to wash the existing finishing layer and refinish it with the correct process. It is a costly, big job; therefore, this problem should be avoided. Here are some things that are needed to ensure that the finishing results are of good quality.

• Substrate preparation

Preparation of the substrate is one of the keys to producing a good quality finishing layer. Take the necessary steps according to the type of substrate. For wood and veneer, do good sanding according to the required standards. Check the MC and make sure it meets the safe standards for the finishing process.

For engineering wood materials that have a slick and hard surface, such as MDF, particle board, HPL, LPL, and decorative paper, you need to sand their surfaces to open the surface and make the scratches needed as a bond with the finishing material that will be coated on top.

For natural materials that contain wax, such as rattan, water hyacinth, and pandanus, you need to clean the surface of the wax layer. Use a strong solvent to rub and wash the surface before applying the finishing material on them. Special finishing materials may be used to strengthen the adhesion of the finishing.

For wood that undergoes a chemical process such as bleaching, fuming, or other chemical process, then you have to make sure there is no chemical residue left on the surface. Clean the surface with water or neutralized chemicals to clean the surface before the finishing process begins.

•  The right finishing process

The right finishing process is needed to produce a good-quality finish. Use good-quality finishing materials and make sure each process is carried out properly by following the technical data recommended by the manufacturer. Use finishing materials that help adhesion for some problem-prone finishing.

Conduct testing every time you create a new color panel, especially when you do new material, substrate, or system. Every substrate or material may have specific characteristics that need to be understood to get the right process. Make sure your finishing process is safe before running it in the big volume production. 

• Control the finishing material

Use good-quality finishing materials. Usually material problems are detected when the material is being used; at this time the repair process will need time and energy, which is costly. Therefore, you better choose finishing materials from a reliable company to minimize the risk of problems resulting from finishing materials. Keep and handle the material properly, according to the technical data from your supplier. Check on all materials before they are used; make sure you use only the materials in good condition.

• Pot life

If you used the catalyzed coating, then you have to be aware of the pot life. The finishing materials that are dry with chemical reactions, such as PU, melamine, or 2-component finishing materials, have a certain lifetime (pot life). When the material has been mixed with its hardener, it must be applied immediately to let the material react and dry when the material is layered on the surface. Don't apply the material that has already reached its pot life. Materials that have passed the pot life period mean that they have finished reacting and cannot adhere well to the surface they are coated with.

• Follow the drying time of each finishing steps

The solvent finishing process that uses oil-based stain or glaze has the risk of getting the oil solvent trapped in the coating layer. If the coating is applied on the glaze that has not dried, then the oil will be trapped under the coating. The oil will adhere to the bond between the film layers  and result in adhesion problems.

A similar case is when the finishing system combines the water-based and solvent-based materials. Make sure you follow the drying of each finishing step. The presence of water trapped in solvent-based coating or the solvent trapped in the water-based coating will interfere with the adhesion of the whole finish. 

• Testing and observation

Every time you develop a color, especially for new finishing processes, it is highly recommended to conduct a finishing test before using it. There is a standard test for the finishing quality, but you may need to do some specific tests according to the uses of the product. Some products may need special requirements that you have to acknowledge before deciding to make them. Make sure the finishing system you create is safe before being used for the production process for large quantities.

If you are doing a mass production process, then conduct periodic testing to see the quality of the finishing in the production process. Run several panels in the production line for sample testing purposes to ensure that all products produced have the expected quality.