Bubble and Pinhole in the Wood Finishing

The bubbles in the finishing layer is happening because there is air trapped and bubbled in the finishing layer. This bubble problem occurs in coating applications, finishing materials that form a film layer. These bubbles can appear at every stage of coating application, starting from sanding sealer, topcoat, or even base coat or primer.

Pinholes are tiny holes (like dots) on the surface of the finishing layer that resemble needle puncture marks. Pinholes and bubbles are problems that most often occur together; most pinholes appear due to bubbles. Bubbles that break will produce pinholes; bubbles that are sanded and coated with coating will appear as pinholes.

Bubbles and pinholes become a problem in finishing because they ruin the appearance of the finishing, especially for close-pore finishing or finishing with a thick film layer. Pinholes often will not be a problem in open pore finishing because they will be camouflaged by the natural texture of the wood pores and grain. Bubbles that appear on open-pore finishing can be handled by good sanding followed by the application of coating; the bubbles will disappear and become pinholes. In many cases, some pinholes are camouflaged by the wood grain and pores. However, if this problem occurs on finishing with a thick film layer, especially with dark color and high gloss, then the bubbles and pinholes will be clearly visible and greatly damage the appearance of the finishing.

Pinholes and bubbles are complicated problems that will be difficult to fix; therefore, it is better to prevent and avoid them before they happen. Here are some things that can be done to prevent the emergence of bubble problems.

pinholes and bubbles on the dark color finish

Causes of bubbles and the solutions

• Finishing materials quality

The problems in the material will directly affect the finishing quality. The bubble problem is the film problem then it is mainly depend to the coating and paint quality. Most bubbles problem is caused by the low quality of coating, that are : sealer, top coat, base coat, primer coat, enamel and other film former materials. The other materials such as stain, glaze or filler also can cause bubbles if they are contaminated or applied incorrectly.

There are several causes that make the quality of finishing materials not good, such as the bad formulation, the improper mixing and handling process, expired material, contamination, incorrect material selection, etc.

Solution:

Make sure you only use good quality materials. Using problematic materials will only invite problems later on. Make sure you check the materials you are going to use, apply them to a small panel, and see the results before using them on a large product.

Use, mix, store, and handle materials according to the manufacturer's instructions. Read the technical data instructions of the product carefully and follow them. Understand each type of coating and use them according to the substrates and the desired finishing quality. Make a step panel to define the finishing process before running the finishing for the production and big product.

• Coating layer that is too thick

Applying a coating layer that is too thick will make the releasing thinner from the film layer become slow and difficult. This condition risks air or water vapor being trapped in the film layer. It also makes the film drying take a long time; the film layer formed cannot dry perfectly. When it is coated with another coating, then the film keeps releasing its solvents and risks forming bubbles.

Solution 

It is highly recommended to apply coating with a reasonable thickness; avoid too thick film coating in one application. Do a wet spray of 1 or 2 layers on each coating (paint) application, wait for the coating layer to dry, sand it well, then apply another coating on it. Repeat this process to build the thick coating. It is very important to follow the drying time of each coating layer and properly sand for each coating layer.

Use the correct spray application technique to prevent coating application that is too thick. Set and adjust the spray gun correctly and do the spray movement correctly to produce an even coating layer. See our previous post: Spraying technique to the finishing

• Imperfect drying of the coating layer

Bubbles can also be caused by the imperfect drying process. The solvent or thinner that is still in the coating will continue to try to get out of the coating layer, and when it is trapped in the film layer, it will trigger bubbles.

solution

Use thinner with good quality that is compatible with the coating material types. The proper thinner will make the solvent-releasing process take place properly. Avoid using thinner that contains too much diluent, which will only make the drying process harder

• Undried oil stain or glaze

In the finishing process that uses wiping, brushing, or dipping stain, mostly the stain used is an oil-based stain to make the stain application easier. If this wood stain is still not completely dry, there will be oil left on the surface. The oil residue on the surface that does not dissolve with the coating can also trigger bubbles.

Solution

If you use oil stain or glaze, make sure the glaze and stain are completely dry before being coated with a layer of coating on top. Glaze and wiping stain are generally designed with a longer drying time to facilitate the application process. However, it is necessary to ensure that the stain and glaze are completely dry when it is coated with paint or coating. Check the recommended drying time of the stain and glaze before using it in the finishing process. 

• Room conditions, too dry or to hot

Bubbles can also occur because the finishing room conditions are too hot or too dry. When the air is hot and dry, the drying process takes place very fast so that there is a risk of air being trapped in the film layer.

Solution

Always check and control the temperature and humidity in the finishing room. Make sure the room conditions are in accordance with the coating recommendation. Adjust the humidity and temperature of the finishing room according to the coating specification. In very dry room conditions, you can wet the room floor with water to lower the air temperature and increase the humidity of the room.

Some adjustments can be made to the coating material to make it dry faster or slower to prevent bubbles. The selection of solvents and the addition of additives can be done to speed up or slow down the drying process.

• Wood pores and grain

Some types of wood have small but deep fibers and pores, such as mahogany, nyatoh, okume, etc. These woods have a high risk of getting bubble problems, especially when you do close pores finish.

Solution:

Recognize and understand the types of wood; make sure you can choose the right finishing process according to the type and character of the wood. Apply good wood filler to porous wood to create a close-pore finish. Wiping filler application can be used to fill small and deep pores and grain and prevent bubble problems

• Chemical contamination

Some finishing processes use chemicals to make and adjust color, such as bleaching to whiten the color or other chemicals to color the wood. The presence of chemicals on the surface will interfere with the adhesion and drying process of the finishing layer. The chemicals that still react with the wood will cause bubble problems in the finishing layer

Solution:

If you use a chemical process for wood treatment and coloring, then make sure there is no more chemical residue on the wood surface when the finishing process is carried out. Neutralize and wash the chemicals carefully, and make sure there is no chemical left on the surface. Some chemical are solved in the water; make sure you follow the drying process needed to ensure that the wood is already dry when you do the finishing process

How to fix with bubbles in the finishing layer

The presence of bubbles and pinholes in the finishing layer must be cleaned to make the clean and even finishing surface. One way to clean bubbles is to do good sanding and then coat it with a good coating layer. The bubbles that are there when sanded and coated with a thin layer of coating will become pinholes. A few pinholes on the finishing surface may still be acceptable, especially when the open pores or semi-open pores finish is built.

If the remaining pinholes still look disturbing, then you need to remove and clean them. Wait until the coating layer is dry, do a good sanding, coat it with clear coat. Check whether the existing pinholes are covered by the sealer layer. If the pinholes are still there, repeat the process until all the pinholes are filled and covered by the coating layer and a smooth and even finishing layer is formed. This process requires precision and patience; make sure each sanding process is carried out properly, without damaging the color of the finishing layer, but can clean the pinholes on the surface. The coating application process must also be carried out carefully; avoid too thick a layer because it risks causing bubbles again. 

If the sanding and coating application processes cannot solve the bubble or pinhole problem, then the only way to fix this problem is by washing and refinishing. Bubble problems caused by coating quality can only be solved by removing the finishing layer and refinishing with the correct process and materials. Bubbles caused by chemical contamination on the wood surface can only be cured by removing the finishing layer, cleaning the chemicals on the wood surface, and the refinishing process.