Like all substrata, wood also needs special care in its preparation. Surface preparation for painting is essential for obtaining a good finishing. The surface should be fully dry. All oil or fat stains should be removed with solvent, carrying out strict sanding with sandpaper of adequate grain, and completely removing dust with dry or slightly humid cloth. For preparation, sealing or primer coating is necessary for wood, in order to prevent the wood from absorbing too much paint.
The sanding process is essential for substratum preparation. Only after an adequate sanding does finishing reach its two objectives:
1- To preserve or protect the wood;
2- To increase the beauty and enhance the grain of the wood.
In general, the reason for sanding is to turn wood softer, removing splinters generated by the machines, teeth, depressions, and other surface imperfections.
Furthermore, it is necessary to sand the entire surface immediately before the application of the finishing, since the wood is hygroscopic – it absorbs atmospheric moisture, causing swelling of the fibre, leading to a fully uneven surface. If it is not removed, this will enhanced with the finishing process.
The sanding process can be performed using a variety of different kinds of machines. Each kind of sanding machine has better performance in the specific task. It is worth noting that work speed and finishing refinement also depend on the kind of sandpaper used. In general, the higher the sandpaper’s number the more perfect will be the result.
During the finishing process, additional sanding is performed, generally after applying the sealing, particularly for removing the fibres that result from moisture absorption along the surface.
When the wood is exposed to the air, which has high quantity of variable humidity, wood moisture content is rapidly altered. Such change occurs particularly transversally to the wood grain and there is very little absorption along the grain direction.
The care with which the piece’s drying was performed is important. If moisture conditions alter, the piece shall absorb humidity, settling itself in the new atmospheric condition. If wood moisture content is greater than the equilibrium of a given relative moisture, the wood shall lose moisture and shrink in volume.
However, if the wood is dryer than the equilibrium of a given relative unit, it shall absorb humidity and swell. Wood shrinking and swelling movement varies from species to species and with wood grain direction. This occurs, because the wood structure is different for each species.
Wood log shrinkage or swelling quantity is twice larger along the tangential surface than along the longitudinal surface. Consequently, during drying, a humid piece of wood cracks more in a direction than in another and tends to twist.
In general, crack quantity is roughly proportional to the loss of moisture below the fibre saturation point. Such difference in the crack trend means that when wood veneers with different moisture contents, of different wood species or different grains are placed side by side, for the composition of a plywood piece or for the composition of a table lid or solid wood or solid wood chest of drawers, some parts shall shrink or swell more than others, due to change in moisture content, resulting in a fully irregular surface due to different expansion and contraction behaviours with the changes in moisture content.
Moisture content in the manufacturing and finishing stage shall be approximately the average of moisture that eventually the piece shall be exposed for use, or slightly smaller. Air moisture around the piece in use guides the time that the piece shall be dried.
Variation in moisture content is considerably reduced with the application of paint or varnish coatings. It is common to observe that when a finishing in a piece cracks, due to the change in moisture content, it normally begins along the edge or patch without paint, and is most severe next to the edge or patch than in the central region.
Economic finishings can be applied on the back of the furniture pieces, in order to protect the surface against. Normally, 1 or 2 coatings are applied, depending on the resistance to moisture desired.
Any change in the moisture content shall result in surface movement of the wood under the finishing and if frequently repeated, this contraction and expansion movement can seriously damage the finishing.
Wood moisture content should be observed, before its finishing, as well as relative moisture and the temperature of several departments in the manufacturing process, stocks of goods and transport. Wood that is too dry in the finishing stage is as bad as wood that is too humid, since in both cases, any change in moisture content shall cause wood movement.
Source: Elaborated by the Journalistic Team of the Magazine of the Wood