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CLOSE THIS BOOKSurface Finish by Dyeing, Staining, Pigmenting, Matting and Polishing - Course: Manual woodworking techniques. Trainees' handbook of lessons (Institut fr Berufliche Entwicklung, 22 p.)
3. Coating Processes
VIEW THE DOCUMENT3.1. Basic Terms
VIEW THE DOCUMENT3.2. Matting
VIEW THE DOCUMENT3.3. Varnishing
VIEW THE DOCUMENT3.4. Polishing

Surface Finish by Dyeing, Staining, Pigmenting, Matting and Polishing - Course: Manual woodworking techniques. Trainees' handbook of lessons (Institut fr Berufliche Entwicklung, 22 p.)

3. Coating Processes

3.1. Basic Terms

Lacquers or varnish paints that dry physically or harden chemically form a solid coating film improving the wood surface appearance and protecting it against mechanical and chemical influences. Stained or natural-coloured surfaces require transparent coats, so that the wood texture appears clearly and distinctly. The varnishes used are solutions of solid substances (natural or artificial resins) that, after being distributed are drying or hardening and thus, forming a solid film on the wood surface. The coating film consists of one or several layers to be applied shortly one after the other and partially "wet in wet" or after intermediate drying, respectively.

Coat layers

1st)

Priming coat:


Priming agents, fillers or pre-stains even up slightly rough surfaces, close wood pores and form a pre-requisite for the varnish to adhere properly to the wood.

2nd)

Intermediate coat:


Varnish layer thicknesses are enlarged and adhesiveness between prime and final coats are improved by priming paints or interlayer varnishes.

3rd)

Finishing or top coat:


The required surface effect or the envisaged colour is obtained with matting agents or glossy varnishes.

Coating of surfaces

With open pores:

Pores have not been filled, but internal walls of pores have been coated with coating film.


Figure 14 Surface with open pores 1 board, 2 coating

With semi-closed pores:

Pores have been filled only partially, while the remaining surface has been coated with a thin coating film.


Figure 15 Surface with semi-closed pores 1 board, 2 coating, 3 pore filling

With closed pores:

Pores have been completely filled and are covered by a closed coating film.


Figure 16 Surface with closed pores

The desired porosity is obtained through various coating methods:

Matting:

Surface coated with transparent coating film. Its final appearance is yielded by a finishing procedure. Surface remains with open pores and is dull to dull-lustre.

Varnishing:

Surface coated with transparent or pigmented material. No further treatment of the surface. Surface has closed pores and is dull or shining, depending on the varnish constitution.

Polishing:

Surface coated with transparent or pigmented material. Final surface effect is reached not until after further operations (e.g. buffing or polishing with a cloth) and may be high-gloss, slightly polished or dull.

According to the finishing methods applied and the coating materials used, the following gloss grades are distinguished:

high-gloss:

Surface reflecting objects approximately without distortions.

dull-shiny:

Surface reflecting objects only unclearly and in a subdued manner.

slightly polished:

Surface reflecting objects distortedly or unclearly.

dull:

Surface reflecting objects not at all or only weakly and blurredly.

What is the task of cleanly carried out prime coats?
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What is the porosity difference of matted surfaces compared to varnished ones?
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3.2. Matting

All varnishes dry up with a special shine. In order not to produce a mirror like gloss with certain varnishes, but dull and silky lustre surfaces, matting agents are added to the varnishes. Such agents are finest ground substances (inorganic or organic compounds) added to the varnish at the amount of 5 to 20 %.

Coating operations:

- Spread the highly diluted matting agent with a paintbrush in grain direction at ambient temperature.

- Slightly grind the hardened prime coat with fine sandpaper with your LI LL

- Brush the surfaces until dust is removed.

- Apply hardly diluted matting agent with a cloth in wood grain direction until a uniform dull surface is obtained.

Recommendations:

- When wiping the surface, avoid touching the same spot twice, otherwise irritating glossy stripes will appear.

- If several coats are applied, the commensurate previous film must be properly dried, otherwise it may rise up and be torn off.

Along with the manual coating the mechanical spraying method may be applied.

Spraying operations:

- Prepare varnish, ensure its proper temperature and mix it. Make sure if the varnish is sufficiently diluted.

- Select spraying gun nozzle opening of about 1.8 mm; air pressure of about 3 at. overpressure; nozzle distance from surface of 250 to 300 mm approximately, hold spraying gun at an angle of about 45°.

- Spraying ensues cross-wise (i.e. "with the grain" and "across the grain"), pay attention to the surface edges.


Figure 17 Crisscross spraying

Recommendations:

- Check varnish vessel for safety.
- Filter varnish, if required.
- Check whether compressed-air connection is leakproof.
- Apply a test coat.
- Select round jet for narrow faces and flat jet for wide faces.
- Observe respiratory protection while spraying.

What has to be especially observed when the matting agent is applied to the surface?

3.3. Varnishing

"Varnishes" are liquid coating agents without dyestuff additives. After drying they form a well-adhering film of particular gloss effect Despite of the partially yellowish or brownish colour tone they are transparent. "Varnish paints" are varnishes which pigments and dyes are added to.

Varnishes and varnish paints are mainly used as:

- Shellacs

for house furniture, casings

- Oil and alkyd resin varnishes

for kitchen furniture, garden furniture, windows and doors, wooden structures

- Nitrocellulose varnishes

for interior furnishings, house furniture, rack and shelf furniture, toys, casings

- Acid-hardening varnishes

for interior furnishings, house furniture, rack and shelf furniture, kitchen furniture, office and school furniture, garden furniture, casings, windows, doors, parquet

- Polyester varnishes

for interior furnishings, house furniture, kitchen furniture, casings

- Polyurethane varnishes

for interior furnishings, all kinds of furniture, doors, staircases, parquet

Varnishing ensues in a making up process:

After the first varnish coat has dried and a possible intermediate grinding has been carried out, the next varnish coat is applied. This procedure is repeated several times until the required quality is obtained. Commonly used coating methods are, above all, painting, spraying, rolling, dipping, pouring.

Recommendations:

- Previous oiling of wood surfaces may cause damages, because oil is seldom compatible with coating agents.

- Wood to be covered with coating agents must have moisture content of 8 to 10 %. More moisture would ooze to the surface later on and destroy the varnish film.

- Prior to the coating of wood surfaces, varnishing tests are to be carried out, in order to find out the reaction of the materials.

- Prime coats are to be made as lean as possible (highly diluted), with each subsequent coat having to be made less diluted than the previous one.

- The required drying spells of 6 hours must be adhered to following each spraying operation (except in case of "wet-in-wet" application of polyester varnishes).

- Wooden workpieces the surface of which is coated with polyester varnishes, have to be laid down horizontally for at least one hour, so that the varnish film will have a uniform thickness and does not run off. The second varnish coat should be applied "wet-in-wet" after 20 minutes, - just at the moment when the previous coat begins to jelly.

- After the final coating with nitrocellulose varnishes, a drying period of 4 days is needed.

Which damage may be caused, when too damp a wood is varnished?
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3.4. Polishing

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

Smoothing process:

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 processes?
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Why must the first cover coating (the same applies to intermediate polishing too) be always applied without polishing oil?
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