Finishing defects - Cracks in the film

Sometimes, after the varnish has completely dried, small cracks in the film may be visible. This is a finishing defect that can have various causes. Unfortunately, to solve the problem, the cracked coating must be sanded away and a new one applied.

What can cause the defect.

Thickness of applied layer. Data sheets and technologies usually specify the grammage to be applied to each coat. When the applied layer is very thick, it creates stresses in the film that lead to cracking. Also specified in the data sheet is the number of coats that can be applied. More than the recommended number leads to a thick film that can crack over time. Sometimes, when the defect is found, the film is lightly sanded and another coat applied. This is not the best solution because it thickens the varnish and increases the risk of cracking. Vigorous sanding is recommended to remove as much of the problem layer as possible. The maximum total quantity applied should not exceed 400-450 g/ m2 . At higher quantities there is a risk of cracking.

Temperature and humidity combined with the use of a rigid material. Large variations in humidity lead to dimensional variations in solid wood. If the varnish is not sufficiently elastic or the layer is too thick, cracking can occur in the joint areas and along the grain. When working with solid wood it is best to use more elastic products with less filler and more resin. They are more expensive, but the cost of repairing cracks is infinitely higher.

More catalyst than recommended. In catalyzed products, cracking may occur if more catalyst is added than recommended. Catalyst is not an elastic product and as the amount of catalyst added increases, the film becomes brittle.

Using a varnish with expired pot-life. The pot-life of the varnish-catalyst mixture is specified in the data sheet. After this time the material does not harden instantly. It continues to remain fluid for a while and gives the impression that it can be used. In reality the varnish loses its properties and the resulting film may crack.

Applying a new layer before the previous layer is completely dry. In the case of wet-on-wet application, the next coat is applied after the solvent has evaporated, i.e. 30-45 minutes after application.

Incompatibility between finish coats. If refinishing or refreshing a piece of furniture, check the compatibility of the materials used. If the layers are not compatible, cracks will appear. Sometimes this can be an advantage. Crackle or crackling effectis based on the incompatibility of two successive layers of material.

Combination with some substances in the previous layers or in the substrate (wood, MDF) can lead to cracking

There are topcoats where the technical data sheet states that no more than one coat can be applied. There are substances inside the varnish, usually waxes, which give a very nice appearance to the topcoat, but do not accept another coat on top. When defects appear, sanding must be done very thoroughly before reapplying the final coat.

Sometimes cracks appear on the edges of MDF. It happens when there is incompatibility between the substances in MDF and varnish. It is therefore recommended to sand the edges and milled surfaces and protect them with special insulating materials.

In general, the cracks caused by dimensional variation in the wood are at the joints or along the grain, parallel to the grain. When it is the lake that is to blame, the cracks are in the form of a lattice. For example, if the watertight watertight varnish freezes, it loses its properties. After thawing, it can no longer be used. If it is used, star-shaped cracks appear after drying, similar to the cracks that appear on mirrors or glass when a marble or similar object falls on them.

I hope you find the information useful. As always, additions are welcome. If you have any questions or queries, please leave them below in the space provided. I will be sure to reply. And if you have encountered other problems, try to identify the cause with the links below to other finishing defects.

Orange peel

Air bubbles. Stings in the film

Craters (fish eye)

Leaky lake