Wood panels are usually coated with varnishes or transparent paints. This type of finish does not hide flaws and shortcomings, and even highlights them. To properly proceed with the elimination of defects or to alleviate them, you need to know their type and frequency of occurrence.
Knots are among the basic and most common wood defects. A knot is the remains of the base of the branch growing from the trunk. As a result of breaking or cutting off a branch, the adhesion is released and the so-called. a knot falling out (drawing).
Drawing. The formation and types of knots: a) fused knot, b) part of the knot has grown up, c) part of the knot falling out
Smaller, fused knots are not considered defects in wood panels, especially for such species, like a larch, pine, spruce and Douglas fir, provided that they do not appear at the edges. From an aesthetic point of view, some healthy knots can be used as ornaments. In solid oak wood panels, ash tree, all knots must be removed from elm and birch trees. How to use knotty wood will be described in the article on solid wood panels. When processing knotty boards, especially for planing and milling, it is important not to tear the wood fibers close to the knot. These places cannot be repaired later. Greater attention should also be paid to smoothing, after which there should be no dimples and unevenness in the knot and the area around the knot.
Knots falling out, if they are not black, spoiled or with a thin layer of bark, can be pasted into the hole, especially knots of larch and spruce. Pasting knots is, from an aesthetic point of view, better than drilling holes and inserting wooden pegs in their place, the so-called. traffic jams. Large black knots, spoiled must be removed.
Cracks are also a very serious disadvantage of wood. Frontal cracks in sawn timber 1–5 cm deep must always be cut off. Smaller internal cracks can be filled with glue and the wood pressed together with clamps. In this way, the crack can be completely eliminated. Material with deep and numerous cracks is basically unsuitable for processing.
Resin blisters are flat, resin-filled cracks in larch wood, pine and spruce. When the cut passes through the bladder, resin is leaking from it, this often happens even after varnishing or painting. Therefore, it is recommended not to use kerf planes with more resin blisters in visible places, e.g.. as front paneling planes, etc.. Smaller resin blisters can be cut with a sharp chisel along with the surrounding wood. The resulting groove is filled with a wood putty, slightly darker in color than the wood (the wood darkens over time). However, it should be noted, that you will almost never be able to choose the shade of a putty for wood and that you should take into account.
The unnatural color of the wood is the result of fungi. The initial stage of rot is characterized by a change in the color of the wood, which usually does not mean a deterioration of the mechanical properties. Drying the wood to the proper humidity stops the decay process and further discoloration. However, discolored wood is not suitable for panels.
Pine is especially sensitive to discoloration (the so-called. sinus). Blue pine wood for panels is not suitable. When and how pine wood turns blue? Most often, bruising then occurs, when unbarked pine logs are kept in a damp place for a long time or under similar conditions, without spacers, lumber is stored. This defect can be prevented by correct positioning, with a roof, stacks of lumber, and thereby drying to a humidity below 20%.
White is brighter, the circumferential part of the trunk of most tree species (just like the darker one, the middle part, called tough guy). The thickness of the whitish layer ranges from 2 do 10 cm and depends on the species and age of the tree. In some species, e.g.. oak and robin white should be considered a defect and should be removed at least from the visible side of the panels. Larch wood should also be free of sapwood, because it is much lighter than the tough guys. In practice, however, larch wood is rarely devoid of sapwood. In any case, when lining surfaces outdoors with larch wood, whiteness should be removed. Pine sapwood, on the other hand, is used in the same way as heartwood.
Places damaged by insects must be removed, especially from lumber dried in natural conditions. Lumber dried in drying chambers is devoid of live insects and their larvae. However, this lumber can be attacked secondarily by insects, just like dried naturally. The visible sides of the wood panels should not, however, contain traces of insect corridors.