|Surface Finish by Dyeing, Staining, Pigmenting, Matting and Polishing - Course: Manual woodworking techniques. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 22 p.)|
|3. Coating Processes|
During the polishing process rough, dull and uneven surfaces are treated so that they will become extremely smooth and high-gloss. The objects being reflected on the polished surfaces should be mirrored as undistorted as possible. High-gloss surfaces must have a permanent shine - without any streaks or greyish shades. In this way the decorative aesthetic effect of the wood texture is particularly emphasized.
If the humidity in the polishing rooms is higher than 60 %, surface defects may be expected (greyish shades and "clouds").
If it is positive that a wooden workpiece is to be polished, it must be ensured that there are not any protruding edges on the wooden structure, so that the polishing balls and brushes can slide on the surfaces without interruption. If this condition cannot be fulfilled, the individual parts must be polished before being assembled. Wood with fine pores is especially suited for high-gloss varnish coats. Should, however, wood with large pores be used, its pores would have to be filled prior to the varnish application. Polishing always necessitates a closing of the wood pores with transparent fillers.
Making up process:
After the wood pores have been closed, the polish varnish layers are generally applied coat by coat manually until a film thickness is obtained, showing the desired shine.
Figure 18 Making up process 1 board, 2 varnish layers individually applied
Film thicknesses excessively applied by spraying, rolling or pouring are removed by mechanical fine-grinding (buffing) with special grinding means and polishing brushes until the high lustre is obtained.
Figure 19 Smoothing process 1 board, 2 varnish layers individually applied, 3 polishing brush, 4 varnish coat polished down
The varnishes used should have special properties. They must be resistant to heat and moisture, drying quickly and well grindable. They must not be brittle or sticky. Such varnishes must be labelled as being able to be sprayed on and ground.
Polishing rules for shellac polishes
- Polish completely dried wood only.
- In order to visually underline wood texture sparingly distribute drying oils on the surface - not too strongly, otherwise oil spots appear later on and the pores cannot be closed any more. Use pumice > I L Id polishing agent to fill the pores.
- A better priming may be effected with cellulose compounds, with the danger of oil bloom being avoided.
- Rub in pore filler at pressure (dyeing pumice with adhesive, diluted polish matching the polish).
- Use light-resisting colours to dye the pore fillers.
- Apply shellac polish coat by coat, let it dry inbetween and grind and smooth it always.
- Apply the first cover coating (the same applies to intermediate polishing too) without polishing oil in order not to include the polishing oil in the polish, by circularly moving the polishing ball. First, use coarser, then finer linen for the polishing ball.
- As for further coating, polishing oil must be added moderately to prevent the ball from aing to the surface. Let enough time pass for drying.
- After the drying process, intermediate polishing has to be done to make the surface resistant and to close it.
- Wipe the polishing ball uniformly and without interruptions.
- In order to remove the polish oil from the surface, final polishing has to be done. For that purpose, drops of styrax benzoin solution are to be sprinkled onto the working surface of the polishing ball. The gum benzoin takes the polish oil up to the surface top. Subsequently, remove the polish oil with diluted sulphuric acid, magnesium oxide or with polishing down solution.
Polishing down agents
After the lacquer-coated surfaces have been given a high-gloss finish through polishing or other polishes have been completely polished down, the oil or polish wax residues must be removed in a final operation. Polishing down agents may also be solutions enriched with finest abrasive. As for shellac polishes, benzo dyestuff serves as polishing down agent. For that purpose, gum benzoin is dissolved with alcohol. It has an anti-rot effect and improves the elasticity of shellac polishes.
What is the difference between the smoothing and making up
Why must the first cover coating (the same applies to
intermediate polishing too) be always applied without polishing