How the quality of the varnish film is influenced by the properties of solid wood

Solid wood furniture is highly prized, and some people don't want to have any furniture other than solid wood. Stains, waxes and oils can be used to beautify and protect it, but synthetic varnishes are also used, which are more resistant but more demanding. If certain conditions are not respected, the finish can fail over time, the cause often being independent of the quality of the varnish. Let's take a look at the problems varnish coatings can have on solid wood, what causes them and how they can be avoided or remedied.

Incorrect or incomplete drying of wood leads to varnish film defects

Sometimes on the finished surface of the furniture appearfine cracks along the wood grain. The first reaction is always "it's the lake". Sometimes - quite rarely - the lake can play a part, but in most cases cause is solid wood that has not been thoroughly dried.

To be properly dried solid wood must be left in the kiln, respecting the specific drying time and cycle, until it reaches equilibrium at a moisture content of 8-12% for wood used indoors. This is for forced drying. In the case of natural drying, the timber must be left to dry for at least 9 months in the stack, during which time water is removed from the wood until it reaches equilibrium moisture content.

Forced drying (by increasing the temperature in the kilns or keeping the wood directly in the sun) can cause cracking, but also a differentiated moisture content in the wood, higher inside and lower outside. Over time, the wood will try to balance its moisture, and during this process it will move. If this movement continues after finishing, the varnish will 'feel' it and if it is not elastic enough, fine cracks may appear along the grain throughout the varnished surface.

This is why outdoor varnishes are very elastic, even if this is at the expense of hardness. Outdoors, where the wood should have moisture in the range 14-18%, the possibility of the wood being influenced by the variation in humidity in the atmosphere is much greater. The result is a movement of the wood which, in the absence of an elastic varnish, would lead to cracks in the veneer.

Bad sanding is the cause of many finishing defects

Finishing defects that originate from the correctness with which the wood is prepared before varnish application are lack of adhesion, differential absorptionand tarnished appearance. All this is caused by incorrect sanding of the wood before the finish is applied.Polishing should be done in stages, using increasingly fine sandpaper. Also, the sandpaper should be changed when it gets dusty or when the sandpaper sandpaper starts to fade. If the wood is not sanded well the raised grain is not broken but pressed, forming so-called mirrors. In these areas the wood does not absorb properly, resulting in stains or lack of adhesion.

Defects can also occur due to differential dye uptake in splice areas (straight or finger joints), where there are fibre ends that absorb more dye (even 100 times more). This makes the splice stand out, which is not accepted by everyone. Differential absorption can be avoided by using an isolator before applying dye. A solution may also be to apply a very thin primer, after which the dye is applied.

Poorly chosen construction solutions can also have an effect on the varnish film

Sometimes the constructive solution for wooden doors is to embed a board in a frame. The battens do not need to be glued in place precisely to allow the wood to vary in size if the humidity outside drops or rises. If the skirting were glued to the frame, the system would become very rigid and movement would be much more noticeable, which can lead to cracks in the varnish film. However, it should be borne in mind that in very low humidity (as can happen in winter) the frame may shrink and the previously hidden part may become visible. If it is not finished in the same way as the whole door, that part will stand out. To avoid this the edges of the frame should be stained before it is put into the frame. This is a tip we add to the others about staining, varnishing and painting of wooden doors.

The application technology must be in accordance with the wood used to avoid the appearance of film defects

In the case of large-pored wood such as oak, mahon or nuc, small holes may appear in the film shortly after the furniture has been finished. The phenomenon is also called pore drop. Basically the pores become visible again. If the aim is a very good coating with complete closure of the pores, their rediscovery is not acceptable. "Falling out" can be avoided by using an isolator or hardener varnishes such as polyurethane varnishes. The pore drop phenomenon is especially common when finishing with nitrocellulose materials. Another option would be to apply the first coat of primer very thinly so that the primer "wets" the pores.

Solid wood furniture is not as easy to make as derived wood panels and it is not enough to know how to mechanically process wood. You also need to know about wood species and their behaviour over time, their main characteristics and 'weak' points, drying, the influence of humidity and temperature and other such information. This is useful for producing quality furniture that will last for a long time.

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