Preparation of a wooden surface for varnishing
A decorative wood panel can never be left unfinished. Wood is a porous material susceptible to contamination. Therefore, they should be covered with a layer of varnish, which will protect the surface and emphasize the grain pattern and color. Seems, that painting is a simple activity. In fact, it looks a bit different, especially then, when wasteful to coat the wood exposed to the elements. The situation becomes more complicated then, when defects arise as a result of improper surface finish, which is difficult to remove. For example. blackening of wood under varnish happens very often, however, it is very rare to meet an experienced painter, who could do something about this. Such situations must be prevented.
Preparation of the surface for varnishing.
As already mentioned, the quality of the varnished coating depends primarily on the way the wood is processed. The place where the wood is displayed - inside decides about the method of processing, or outdoors.
A wooden panel used inside rooms must be sanded before varnishing. Before varnishing, external panels can be finished in a different way. Here are some of them.
Cut surfaces. Holiday cottages, gables and attics of single-family houses can be lined with untreated boards (i.e.. tireless). This is an unusual method, but because the untreated surface highlights the wood, as a natural material, it can add charm under certain circumstances. The rough surface should never be covered with oil varnishes, synthetic, epoxy, or also thixotropic.
Gloss, which varnishes give, in this case it would have an aesthetic effect completely opposite to the intended one. Raw boards can be (instead, cover it with a layer of varnish, which is applied 3-4 times to the outer boards.
Pigment varnishes are produced in several colors, e.g.. mahoniu, palisandru, pine trees or greenery, and as transparent. However, clear varnish is not recommended for surfaces exposed to weather conditions. In a short time the wood will start to turn gray and sometimes turn black, which is the result of the action of mold growing under the influence of increasing humidity. Transparent varnishes are used only as a base or to emphasize the color shade of the wood. The varnishes do not hide the wood pattern, and they only change its color. Larch boards, pine or spruce can be toned to the aforementioned shades according to your preferences. A raw structure will be created on the sawn wood surfaces, which mainly in darker shades gives an interesting pattern on large external surfaces.
Before varnishing, the surfaces of the cut boards should be lightly sanded with sandpaper 46 or 60 to remove fibers and splinters, which could break off over time and allow the ingress of moisture.
Planed surfaces are also only suitable for exterior coverings. Fine waves, which arise from the rotating knives of the machine tool, they give a "rough" surface after painting. In addition to fine waves, burrs are formed when planing in the vicinity of knots and in places with opposite direction of the fibers. They can only be removed with a wood grinder. An example of correct and incorrect planing in relation to the course of the fibers is shown in the figure.
Drawing. Planing the surface of the board: a) correct (with the course of the fibers), b) incorrect (against the course of the fibers).
On large external planes, these imperfections do not bother; inside, unfortunately, they are not an ornament.
Sanded surfaces are ideal for any type of finish, but their smoothness depends on the grain size of the sandpaper and the direction of grinding. A properly smoothed surface should generally not be wavy, however, it must be free of any torn fibers in the area of the knots (drawing).
Drawing. The waviness of the surface was formed after planing at different revolutions (wavelength 1 depends on the feed rate, the number of knives and the rotation of the planer head: a) at 3000 rpm, b) at 6000 rpm.
Smooth, only achievable by grinding, the surface is very important both for varnishing, and after polishing.
As a rule, the required smoothness will not be obtained with one grain. Grit paper is suitable for rough grinding 60, for finishing grain size 100.
Plank linings should be sanded as follows. Use grit paper for rough sanding 60 and carry out the first sanding perpendicular to the wood fibers. Second, with the same grain size paper, along the fibers and only the third sanding is done with grained paper 100, also along the fibers (drawing).
Drawing. Sanding panel boards: I,- first sanding across the grain with sandpaper 60, II - second sanding along the grain with sandpaper 60, III - third sanding along the grain with sandpaper 100 (the so-called. smoothing)
Lining made of veneer boards is sanded twice, only along the fibers, first with grain paper 60 and then 100. The first sanding should be done carefully, because, especially in machine grinding, the veneer may peel off.
A similar danger also exists then, when the veneered elements will be sanded a few days after gluing. As a result of the drying of water from the adhesive, deformation occurs and the veneer may be abraded on the convexities.
Grinding is to obtain a perfectly smooth and clean surface. This should be kept in mind at all times, that any dirt and stains are enhanced by painting. Perfect sanding is required on surfaces finished with high gloss varnishes. These coatings are very sensitive to any unevenness and contamination.