Cracked Finishing Layer Problems in Wood Finishes: Causes and Solutions

Cracked finish layer is a condition when there are cracks in the film or coating layer. The cracks could be very subtle hairline cracks to large and wide cracks. These cracks can appear immediately after the finish dries, within hours, days, or even months after the product is in use. Cracks that appear immediately when the layer dries mean there is a problem in the finishing system or material. The cracks that appeared after long time, may be caused by low durability of the finishing layer or too extreme abuses that overcome the ability of the finishing layer

No matter what kind of cracks, cracks are a serious problem that degrades the quality of the finish. Here in this post, I will explore and share about cracks' problems, the causes, how to prevent them, and how to repair them. 

crack at the coating

The causes of cracked problem

- Too thick of coating layer

Some cases of the crack problem are caused by too-thick coating application. A normal layer of coat will need time to dry according to its drying time. But when the coat is applied too thickly, it requires a longer drying time, much more than the normal layer, and it will continue to dry until it is completely dry. Meanwhile, the drying process is always accompanied by a volumetric shrinkage of the coating. If this shrinkage exceeds the acceptable volume of the film, cracking will occur in the finishing layer.

Therefore, it is highly recommended to avoid applying too thick a layer of coat in a single application. Apply the finishing material at a reasonable thickness according to the manufacturer's recommendations. To build a thick film, you need to apply the coating several times. Apply 1 coat in the recommended thickness, let it dry completely, sand thoroughly, and then apply the next coat. This process may take time, but it will reduce the risk of cracking.

- Too thick a finishing layer

Even when the coat has been applied according to recommendations and each layer has dried within its designated drying time, the finishing layer can still crack if the overall film thickness is too high. Even when the coating layer has dried during the finishing process, the drying process is still ongoing, albeit very slowly. If the film layer in a finishing coat is very thick, shrinkage can be significant, leading to cracking.

Therefore, avoid applying excessively thick finishing coats to wood. Choose the right finishing material based on the characteristics and capabilities of the coat. NC and water-based coats are not recommended for creating high-build film finishes. Generally, the recommended film thickness for NC, acrylic, and single-component coatings is no more than 4 mils. For thick film finishes, you can use thermosetting coatings such as PU, AC, and UV, which are still safe up to 10 mils of finishing film thickness. For thicker finishes, it's best to use casting resins like PE or epoxy, which can coat surfaces with a film layer up to several mms thick.

crack at the whit paint

- Low-quality coating

The performance of a finishing coat is highly determined by the quality of the materials used. A low-quality coat can certainly lead to cracking in the film. Therefore, use good-quality coating and finishing materials and follow the application and mixing process according to the instructions and technical data for each finishing material.

- Improper mixing

A high-quality coat can become inferior if not used and handled properly. Proper mixing is a crucial factor in maintaining coat quality. For single-component materials mixed with solvents, ensure the coat is mixed with the solvent according to specifications, in the correct ratio. For two-component materials that are prone to chemical reactions, proper mixing is very important to ensure the materials react properly and produce the properties that meet product specifications. 

- Too fast drying

Each finishing material requires a specific amount of time to dry and achieve the desired strength. This drying time often presents a constraint in the production process when high production speeds are desired. Therefore, several tricks are often used, including adjusting the drying room temperature, using an oven, and adjusting the thinner mixture. However, drying too quickly can risk problems due to incomplete drying in the inside part that leads to the crack problem.

If you want to speed up the drying process, make sure it's done correctly. Be sure to use methods and techniques recommended by the coating manufacturer. It's best to test the resulting film to ensure the resulting finishing layer performs well

- Improper application

We often combine finishing applications with a combination of coats. For example, NC coats with water-based, PU, AC, or other coatings. If you do that, make sure you do the proper combination; the wrong coat combination can lead to problems, including cracking. The basic rule is never layering the thermoplastic coating with the reaction coating. For more detailed information about this issue, you can read our other post: the combination of coatings in 1 finishing system.

- Problems with substrate movement

Generally, finishing film coats surfaces up to 10 mills (10/1000 in) thick, or approximately 0.254 mm, which is a very thin layer. Therefore, the coating film layer will not be able to withstand movement or dimensional changes of its underlying substrate. If movement or cracking occurs in the substrate, the coating film layer will certainly crack as well.

Therefore, ensure the underlying substrate is stable and does not experience movement when the finishing process begins. Ensure the wood is completely dry according to the standard moisture content (MC) for the wood product. Make sure the construction has proper settings to minimize the risk of wood movement. Using engineered wood such as plywood, MDF, or laminated wood for big and wide panels is recommended to reduce the risk of wood movement. Coat both sides of the wood surface to prevent water absorption from its environment to keep the moisture content (MC) of the wood.

For the casting finish, a thicker film layer can be generated (up to several mm), so slight wood movement may still be resisted by this film layer. However, keeping the product dimension stable is highly recommended for every wood product. 

crack from the wood 

- Extreme environmental conditions

Finishing layers, however, have limitations. If they are subjected to physical treatment beyond their capabilities, they can be damaged. If the finished product is exposed to extreme conditions such as excessive temperature changes, excessive heat, or extreme cold, the finish layer can crack and break.

Therefore, understand and recognize each type of coat, learn their respective strengths, and choose the most suitable coating for the product's intended use. An NC coat, for example, offers the best appearance but isn't weatherproof, making it unsuitable for outdoor products. Likewise, other types of coating have their own strengths and durability requirements, which must be understood. Conduct the necessary tests to determine the strength limits of each finishing material to ensure you choose the most appropriate finish for your needs.

How to repair a cracked finish

The cracks' problem when it happens is quite difficult to repair, especially when it occurs in the product. The easiest way to repair a cracked finish layer is to completely remove the finish layer and recoat. Touch-up processes to repair the damaged areas are very difficult in practice. In some cases, cracked finishes can be cleaned by sanding and then recoating. 

If the crack is caused by the substrate movement, then the substrate must be fixed first. Correct the construction; fix the gap, crack, in the substrate; and make sure there are no more moving parts before doing the refinishing.