Why cracks appear in varnish or paint film

The reasons for this are as follows:

I often see on profile groups the beneficiaries of some works, unhappy that cracks have appeared on the painted or lacquered surface of the ordered furniture. There are also those who have repainted an older piece of furniture themselves and find they have the same problem. In most cases, the problem occurs when the furniture has already arrived at the customer's home or some time after the DIY project has been completed. Unfortunately, fixing it means sanding the affected surface and restarting the finishing process. To avoid this situation, it is good to know the causes of cracks in the varnish or paint film. They relate to the furniture material, finishing materials and technology, environmental conditions, wrong combination of coats, ignoring some manufacturer's instructions and the list could go on. For a simpler presentation we have tried to group them into 5 broad categories which you will discover below.

Applying too thick layers

It is the most common mistake made, both by professionals and amateurs. Professionals, even if they know they only need to apply a certain number of coats of a certain thickness or grammage, may end up applying a thicker layer of material if they have to make repairs. Sanding a layer that has, for example, a pronounced orange peel effect, it does not remove the coating completely and the amount applied may increase. Or try to even out the colour and insist on coloured varnish. And the examples go on. Repainting is simple, thicker coats are applied so that there is no spillage, the paint covers the surface better and the operation is completed more quickly.

The problem is that a thicker layer can crack because stresses build up inside the film. And not only in the case of application on solid wood, where dimensional changes in the wood may occur due to moisture, but also if the application is made on MDF or plywood. In order to avoid the occurrence of defects, the total amount of finishing material applied must not exceed 400-450 g/m².

Dimensional variation of wood

The cracks that appear in the film due to wood movements are recognisable because they follow the wood grain, being parallel to it, or the joint lines of the furniture elements. Causes can be:

• Incompletely or incorrectly dried wood
• coating wood used outdoors with varnishes that are too hard and rigid
• wrong constructive solutions
• very low humidity in the space where the furniture is used.

About the moisture content of wood used indoors and outdoors you can find information here and here. This moisture must be in equilibrium with the environment and if it is higher or lower, the wood will exchange with the environment to establish equilibrium. The greater the difference in humidity from normal, the greater the movement of the wood and the greater the risk of cracking. If such wood is coated with a very hard and rigid varnish, stresses build up and lead to noticeable cracks. This is why exterior varnishes are elastic and less hard, and why oiled or waxed wood performs well in outdoor conditions.

A special situation is when cracks follow the joints of wooden door elements, the contours of floorboards or even gaps appear between floorboards. It usually happens in winter when indoor humidity is very low. It's a good idea to have a hygrometer in your home, in addition to a thermometer, to show your indoor humidity. The normal indoor humidity is 50-60%. Because of the heating equipment, but also because of the low outdoor humidity in winter, the indoor humidity drops and is rebalanced by the water found in indoor objects, including wooden floors, doors and furniture. Humidifiers are recommended to avoid this. If such a situation has arisen it is best not to rush with the remedy and wait for the warm season to arrive. It is possible that the defect will disappear and no further intervention will be necessary.

Incompatibility between layers

Applying varnishes or paints that are not compatible with each other can lead to cracking when the top coat dries out. The cracks are lattice-shaped and cover the entire surface. For example, if you apply an organic solvent-based paint (polyurethane, polyester, solvent acrylic) on top of alkyd or oil-based paint, you will end up with a cracked surface. The paint on top will collect in thousands of islands. It's actually what underlies the effect crakele (cracked, crackingCracks also occur if polyurethane varnish with coatings is applied over nitrocellulose varnish. In this case the cracks are gathered from place to place (in the factory the varnish was said to have bloomed).

For some varnishes or paints the manufacturer recommends applying a single coat. The system consists of several coats of primer and only one coat of varnish/paint. It is possible that the varnish contains wax and applying another coat on top of the one already applied may lead to cracks in the film. If you need to apply another coat, lightly sand the previous coat and wipe it off with thinner.

Areas in need of special treatment

When finishing milled areas of MDF, but also in solid wood in the area of the fibre ends, very fine cracks can sometimes appear. These are areas of increased absorption where lacquer or paint can come into contact with substances or tannins inside. To avoid problems, these areas are sanded very well and sealed with special materials (polyurethane or water-based insulators). Blocking absorption results, in the case of wood, in more uniform staining. Those areas (which craftsmen in factories call "twists") are full of fibre ends that absorb a large amount of berry and so the colour darkens a lot.

The inexpensive way to block absorption is to use an aqueous solution of aracet (PVA adhesive) to be applied with a brush. This is what they used in the past. The method is indeed cheaper, but also less controllable and relies a lot on experience. Too little may not block the absorption sufficiently, and too much may lead to loss of adhesion of the varnish or paint.

Failure to follow technology or work instructions

I always remind that it is very important to follow the instructions for use of materials or technologies recommended by the client. In this case, too, failure to follow the technology and working times can lead to cracks in the film. Here are some examples:

• using too much catalyst in the case of hardening varnishes (type A+B)
• use of varnish + hardener mixtures after the time for their use has passed (pot-life outdated)
• applying another coat before the previous one is completely dry
• storage of water-based varnishes at temperatures below 0° and use after a freeze-thaw cycle.

Using a higher amount of catalyst makes the lake more brittle, more sensitive to mechanical action. The film can shatter like a film of glass if an object falls on it or is hit harder. Water-based varnish used after freezing also looks like a broken glass surface.

The appearance of cracks in varnish or paint film is an interesting subject and there is a lot to be said for it. What is good to remember is that cracks that follow the grain of the wood are also caused by the moisture content of the wood, whereas cracks that extend unevenly in all directions are the fault of the varnish/paint or the application method.

I hope you find the above useful. If you have had such situations, please tell us what caused them and how you solved the problem. And if you have any questions or queries, please leave them in the space below. I will certainly answer them.