Why is the lake breaking?

This is one of the most frequently asked questions in comments on articles or seen on specialist groups. Unfortunately, as simple as the question is, the answer is complicated. Even if it is the varnish that is cracking, the problem may be related to the wood it was applied to, the way it was applied or the environmental conditions, not just the varnish. And a correct answer cannot be given without knowing these details.

Basically, the varnish film on the surface of furniture or other objects can crack for four reasons:

1. Lake quality
2. Problems of the substrate on which it was applied: wood, MDF, veneer
3. How the application was made
4. Environmental conditions

Let's take them one by one.

1. Varnish quality leads to problems causing film cracking

Here too, things are not simple and there may be several causes:

• The lake cracks because the layers are not compatible

Lacquers are of many kinds - water-based or solvent-based, catalyzed or non-catalyzed, with different base resins (nitrocellulose, polyurethane, alkyd, etc.). This characteristic means that these varnishes often behave differently and cannot be applied on top of each other. We say they are incompatible products. For example, a polyurethane varnish cannot be applied over a nitro varnish, and an alkyd varnish can never be applied over a nitrocellulose varnish or other organic solvent-based varnish. In both cases the film will crack immediately after drying. Even if there are situations where varnishes of different nature have no compatibility problems, it is recommended to use successive coats of the same nature. Alternate different varnishes only when you know very well how they will behave or if recommended by a specialist.

• The lake cracks because the layer is too thick

Thick layers of varnish (for that matter, of any finishing material) tend to crack. The defect does not appear immediately, but over time. The rule of thumb in finishing is: better several thin coats than one thick coat.

• The lake cracks because it's too hard

Very hard varnishes, such as parquet varnishes, also generally have low elasticity. They must therefore be protected from strong mechanical shocks (knocks) because they can crack like glass or mirrors. Do not use such varnishes where you know the shocks will be strong. Also, do not use them on wood that is not well dried or that tends to vary greatly in size (move) when the humidity outside varies. This is why varnishes used outdoors or for window finishing are softer and more elastic.

• The lake cracks because it's brittle

The varnish film may become brittle if more catalyst is added than recommended. This is the case with A+B hardening varnishes. The varnish loses its elasticity and becomes sensitive to shocks, cracking like glass even when not hit very hard.

• Lake cracks because it has been stored or transported in improper conditions or because it is out of date

Storage and transport conditions are very important. For example, water-based varnishes should not be stored at temperatures below 5°C. If they freeze, they cannot be used after thawing. For example, when applying an outdoor water-based varnish that has gone through a freeze-thaw cycle, small cracks and opalescent areas will appear on the surface. Equally important is compliance with the guarantee period mentioned on the packaging and the conditions under which the lake can be used after opening the packaging.

In the vast majority of cases where the lake is to blame for the cracks, the cracks are oriented in all directions. This is how they can be differentiated from those that occur due to wood movement, which are along the grain or at joints between elements.

2. The varnish may crack due to the substrate to which it is applied

Cracks due to the backing occur most often on solid wood pieces, but there are situations when they also occur on MDF elements or veneer. Let's see what conditions favour them.

Cracks in the film applied to solid wood parts appear if the wood is not sufficiently dry, if the construction solutions chosen are wrong or if the humidity in the room where the varnished object is kept is very low or very high. It is all related to swelling and shrinkage of wood with the variation in outdoor humidity. The varnish film cannot cope with these movements, sometimes very strong ones, and cracks. The cracks this time follow the grain of the wood or the joint lines between the elements. To avoid problems, it is best that the wood is properly dry, coating should be done on all sides so that contact with the outside is as low as possible, suitable construction solutions should be chosen and more elastic varnishes should be used.

Even though it is considered a very stable material, cracks may also appear on MDF elements. The cause is the poor quality of the MDF, which is more loose and porous. The pores contain air that tends to escape at higher temperatures (drying tunnels, very hot summer days). The air pushes into the varnish or paint film and breaks it down. The cracks in this case are specific, like little stars with irregular rays around a central point (see picture below). The film looks like a sheet of paper that has been pushed in with the crease and torn right at the tip of the pencil. The solution is to choose quality MDF or apply an insulator as a first layer.

In the situation cracks in wood or veneered panelsproblems can occur due to too much adhesive being applied. Glue gets into the pores of the veneer and decreases the adhesion of the varnish film. It comes off easily if hit, even very easily, the area whitens and the film breaks. The cracks look like broken glass. It is a defect that is occurring less and less, veneering is now a much more controlled process.

3. The varnish may crack as a result of the way it was applied. For quality results, the recommended technology should be followed

Even if the lacquer is well chosen, the wood well dried or the MDF of good quality, we can ruin everything if the technology and application instructions are not followed. To avoid cracks in the varnish film you should pay attention to the following aspects:

• Wood sanding must be made so that there is a good adhesion. If the film does not adhere well it will peel off and crack. Sand with wood sandpaper (max.220). Using paper that is too fine clogs the pores and reduces adhesion.
• Mix the components in the proportions recommended by the manufacturer. In case of catalyst varnishes, too much catalyst causes the film to become brittle (if too little is added, the film remains soft).
• Catalyzed lacquers have a time limit in which they can be used (pot-life). Do not use the mixture after this time, especially if it has started to thicken. Diluting it does not solve the problem. The varnish has already lost its properties and the resulting film will be softer or crack.
• Do not apply thick coats! Thin the varnish according to the instructions and apply the recommended grammage. Better 2-3 thin coats than one thick one.
• Respect drying times. If the bottom layer is not well dried you risk the top layer cracking.
• Sand between coats to increase adhesion. If the varnish comes off easily it will also crack easily.
• Do not apply two coats of topcoat if the manufacturer recommends only one. The varnish may contain paraffin or other substances to improve the tint or appearance. Coats applied after these substances have migrated to the surface will not adhere and will crack.
• If you want to change the look of a piece of furniture or repair it, check the compatibility of varnishes beforehand. Apply the new varnish to a more hidden area and let it dry. If nothing happens after drying, leave until the next day and lightly tap the surface. If it doesn't peel and crack, the materials are compatible.

4. Lake may crack due to environmental conditions. Temperature and humidity in the work area influence the quality of the varnish film

Temperature and humidity are important not only when transporting and storing varnishes but also in the work area. The best results are achieved if we have temperature in the range 20-22°C and humidity of 60%. It is difficult to have such conditions if the environment is not controlled, but it is still possible to work with good results at lower or higher values. However, working at very low temperatures, below 10°C, should be avoided as adhesion decreases and the film may crack. Also, at very high temperatures the solvent evaporates quickly and there is a risk that the coating is too thick.

As you have seen, varnish film can crack for many reasons. If you ask such a question and the answer comes without being asked about the type of varnish, how it works, what the varnish is applied to, chances are it's not the right one.

I hope you find the above information useful. As usual, additions are welcome. And if you have any questions or queries, please leave them in the space below. I'm sure I'll get back to you.