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After having preworked the wooden parts, their surfaces are to be checked carefully. Surface that externally remain invisible may have faulty spots, if they do not warp later on. Those surfaces need not be treated. Surfaces which are externally visible should have uniform grain and uniform colour shades as well as be completely free from defects.
Figure 1 Checking the surfaces for
faulty spots 1 fissures and holes in wood, 2 faultless wood
A proper surface finish is only then possible, when all the surfaces are completely clean, especially even and smooth as well as free from holes and imprints.
When staining agents or solvents are applied to wooden surfaces, wood fibres and pore brims of sanded surfaces straighten up again. Imprints caused by pressure on the wood may rise again and have a spoiling effect, too.
Figure 2 Enlarged cross-section of a
board 1 upright wood fibres, 2 imprints
Therefore, wooden surfaces are to be washed with hot water without additives before they are finally sanded.
Procedure:
Washing is done with a sponge being rubbed with slight pressure over the surface. After having dried completely, the surfaces are to be re-ground and de-dusted.
The dry wooden surfaces are sanded with moderate pressure and coarse sandpaper, so that the risen pore brims are not pressed down, but ground off.
Much material is removed by sanding across the wood grain. In that case the surface shows, however, a great roughness.
Only a small amount of material is removed by sanding with the grain. Here the surface is, however, very fine.
Figure 3 Sanding the surface 1
"across the grain", 2 "with the grain"
First grind "across the grain", then "with the grain". Use finer paper for each grinding process. Well ground is half polished.
After being washed and ground, the surfaces must be de-dusted thoroughly.
If surfaces are not correctly de-dusted, bright pores will show up, even after staining, having a detrimental effect. Moreover, adhesion of subsequent varnish coats is affected.
Why are wooden surfaces
washed?
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Which grinding direction results in a fine
surface?
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Why must the surfaces be freed from dust
thoroughly?
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Lightwood, especially coniferous wood, is to be treated in a particular way, since the resin components are water-repellent (staining-agent-repellent) and varnish-damaging. This may result in the appearance of spots later on. Solvents (acetone, fuel alcohol, diluent for cellulose lacquers) or saponifiers (curd soap solution, 10 % ammonium solution) help to deresinify the wood.
Application of solvents:
- Place wooden workpieces on a clean support.
- Apply solvent with cotton wad or sponge in stripes several times.
Figure 4 Applying a solvent 1
sponge, 2 board, 3 felt strip, 4 support
- Rub again with a clean, folded up piece of cloth or with a brush.
- Wash with warm water.
- Re-grind the surface with fine-grained abrasive paper.
Note - solvents are inflammable and detrimental to health.
Application of saponifiers:
- Place wooden workpieces on a clean support.- Liberally apply the solution with a sponge several times and allow it to react for 10 to 15 minutes.
- Strongly brush the surface in wood grain direction.
Figure 5 Brushing the wood surface
1 brush
- Thoroughly wash the surface.
- Rub the wet surface with saw dust and clean it.
- Neutralize with 10% acetic acid and wash down with warm water.
- Re-grind the surface with fine-grained abrasive paper.
Recommendations:
- Wooden workpieces deresinified with solvents are not affected or changed in colour as is the case with saponifiers.- After alkaline deresinifying agents have been used, the surface must be well washed and neutral otherwise undesired changes in colour occur during the staining process.
- Do not extract resin from light-coloured wood species with ammonia solution, otherwise the surfaces will darken.
Why must highly resinous wood species be deresinified before
being
stained?
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What is the advantage of using solvents compared to saponifiers?
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Discolourations and spots having developed during the growth of the wood or due to improper storage, as well as spots and stripes as a result of improper staining can be brightened up again or removed. The bleaching agents chemically destroy tannic acids or dyes.
Basic rule:
Always treat the entire surface.
Procedure:
- Place the wooden workpieces on a clean support.- Mix bleaching agents, e.g. 30% hydrogen peroxide solution with 10% ammonia solution in small quantities as they are usable for a limited time only.
- Liberally apply the bleaching agent with a scraper covered with a white cloth or with a plant-fibre paintbrush without metal parts.
Bleaching agents are toxic and very caustic.
Protect your hands with rubber gloves.
Figure 6 Applying the bleaching agent
1 scraper covered with a cloth, 2 rubber glove
- Leave the foam untouched for some time.
- Brush the surface with a hard brush.
- Thoroughly remove the bleaching agent with warm water.
- Let the material dry at ambient temperature for about 24 hours.
Recommendations:
- Spots that came about by rottenness cannot be removed with bleaching agents.- If bleached wood is not coated immediately, it may darken again due to atmospheric oxygen influence.
Which wooden surfaces have to be bleached prior to subsequent
treatment?
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What is the basic rule for
bleaching?
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Why must the surfaces be thoroughly washed after having been bleached?
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Dyes are dissolved in liquids such as spirit, water, oils, etc. and applied to wood in order to change its original colour tone.
In practice, this procedure is also called "staining", although the mode of action is different compared to the actual staining. When the solution penetrates the material, the colouring agents cause the dyes to precipitate in the structure and on the surface of the wood. The great absorptive capacity of soft spring wood tissues brings about a negative image of the texture:
After being dyed, lighter and softer wood spots become darker than denser and more solid ones.
Figure 7 Negative image of the texture
after dyeing
Most dyes veil the wooden surface, so that the grain does not fully show to advantage. Therefore, dyeing is done in such a case when wooden surfaces without clearly visible texture are to be changed in their original colour tone.
The following colouring agents are used:
- water-soluble acid coaltar dyes
- alcohol-soluble alkaline coaltar dyes
- fat-soluble and oil-soluble coaltar and earthy dyes.
Acid and alkaline dye solutions must not be mixed together, otherwise the dye becomes ineffective.
Procedure:
- Place the wood on a clean support.
- Apply the colouring agent wetly and without interruption in wood grain direction.
- Distribute the dyeing agent uniformly.
- Put the wood aside and let it dry for about 24 hours.
Why are wooden surfaces without attractive texture
dyed?
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When liquids are applied to wood being rich in tanning agent, staining causes a chemical reaction in the wood.
Compared to dyeing, generally no dyes are introduced into the wood.
The chemical reaction starts when different salts combine, when acids and lyes are brought together or when salts are mixed with lyes or acids.
The full formation of the actual colour shade will only end after some time has passed. Staining underlines the wood texture optically and compensates different colour shades in the raw wood. Thus, the natural colour tone due to aging of the wood is imitated artificially.
The chemical deposition of the staining agents is light-fast, rubbing-fast and resistant to water.
The good penetrativeness of the solution, even into the dense autumn wood tissues, brings about a positive structural image:
After being stained, denser and more solid wood spots become darker than lighter and softer ones.
Figure 8 Positive image of the texture
after staining
Types of stains
Stains for double-staining (pre-staining and re-staining):
Due to age, location and uneven distribution in the wood, the tannic content is different in various wood species. When the material is pre-stained, tannic acid is added and evenly distributed.
After drying, a metallic salt stain is applied as re-staining agent, the salt of which develops certain colour shades with the now existing tannic acid.
Different pre-staining agents, together with different re-staining agents, produce various colour shades.
Prepare only as much pre-staining agent as being used a day. Light influence disintegrates the pre-stain.
Stains for single staining:
They are heavy metal salt solutions enriched with additional dyes, but only suitable for tanniferous wood species.
Wood treated with double and single stains must not be varnished with polyester varnishes. Otherwise solidification of the wood would be impeded.
Wax stains:
They are solutions containing undissolved metal salts, dyes or pigments and wax additives.
Due to the wax portion, a dull shine will be obtained, with the grain, however, being slightly veiled by the dye portion. Wax stains are only suited for small-size parts of all kinds of wood, but not for furniture industry.
Staining with single stain:
- Place wooden workpieces on clean support.- Check whether all fittings (locks, bolts, plates) have been removed.
- Apply staining agent in grain direction with great wetness and without intermitting, if possible.
- Again apply staining agent after a reaction time of 2 to 4 minutes, in order to ascertain the envisaged staining effect (Surfaces should still be throughout wet and must not have dry spots.)
- Afrse direction and then in longitudinal direction, so that excess staining agent is sucked up and no streaks being left.
Often squeeze out the paintbrush.
- After drying smooth the surface by brushing with a hard brush
in grain direction. Why are wooden surfaces with clearly visible texture
stained?
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What is the difference in the reaction of dyeing agents compared
to staining
agents?
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Staining rules
- Leave any workpiece, before being worked, untouched for 24 hours, at least, at a temperature of 20°C, so that all the pores are opened and the staining solution can penetrate well. Make the staining agent available at the same temperature.
Figure 9 Making the board and
staining agent available at same temperature of at least 20°C 1 board, 2
staining agent bin
- Use boiled water to prepare the staining agent. If tap water is used, staining defects may occur due to lime and iron substances contained in water.- Remove all the metal parts, before staining, so that changes in colour may not occur.
- Before the staining solution ins distributed, remove abrasive dust from all surfaces with a hard brush.
- Only use paintbrushes or sponges for staining. Paintbrushes should not have any iron parts, otherwise the staining agent would change in colour.
- Broad paintbrushes with well absorbent bristles are suitable to distribute the staining agent.
- Before applying the staining agent, undertake a stain test on a material as determined for the piece of furniture.
Figure 10 Staining test on a material
of the same kind 1 piece of furniture, 2 staining test
- After the paintbrush has been dipped into the solution, let it drip off, in order to avoid spatters during its application.- Brush upright surfaces from bottom to top, so that the staining solution may run down the wettened surface.
Figure 11 Applying the staining agent
to vertical surfaces from bottom to top
- Cross-cut wood surfaces, before being stained, are brushed with clear water, so that they do not become too dark.- Staining utensils should not be used for different solutions at the same time - the stained surface might become spotty.
- Do not return stain residues to the vessel, otherwise changes in colour may occur.
- Keep finish-stained parts away from drying in the sun - water and alcohol evaporate too quickly and prevent the development of the colour tone.
- After the staining agent has dried, soon apply the coat so that atmospheric oxygen does not change the colours any more.
- Store liquid stains in well closed bottles or earthenware bins, as atmospheric oxygen and light can change the colour tone of the stain.
Why is a staining test necessary before a piece of furniture is
stained?
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Why must the sample piece for the staining test be made of the
same material and have the same coating as the piece of furniture
itself?
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Why must upright surfaces be stained from bottom to
top?
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As for pigmenting, wood surfaces are coated with covering dyes or varnishes, with the texture being hardly visible afterwards or not at all.
Pigments are insoluble, finely ground paint panicles of anorganic (metallic oxides) or organic (coaltar dyes) compounds to be added to staining agents or varnishes.
- Pigment stains give a slightly covering coat to the wood surface.
- Pigmented dull or glossy varnishes cover the wood texture completely.
Pigment staining procedure:
- Place the wood on a clean support.- Thoroughly work the pigment staining agent into the surface.
- After complete saturation, the pigment staining agent is to be distributed under slight pressure, so that the covering effect of the coat can be fully reached.
- Give sufficient time to dry.
To obtain a smooth wood surface for varnishing and polishing, the existing surface cavities (pores) have to be filled up.
Figure 12 Enlarged cross-section of a
board with filled pores 1 well filled, 2 incompletely filled, 3 incompletely
filled and badly adherent
Pore fillers to be used are 40 to 50 % of binding agents (dry oils or varnishes) and 50 to 60 % of pore filling powders (pumice powder, finely ground barium sulphate, powdered quartz, etc).
The colour tone of the pore filler is to be selected somewhat darker than that of the wood surface stained. Surfaces that are to be polished must be preferably filled with transparent fillers.
Sequence of operations:
- Brush the surface.- Rub in the pore filler by circularly and at high pressure moving a rough linen ball over the wood surface.
- After drying wipe off the residual filler with a soft rag.
Figure 13 Rubbing in the pore filler
1 pore filler, 2 linen ball
Recommendation:
Before the pore filler is rubbed in, the surfaces are to be thoroughly brushed in grain direction, so that the pore rim tips are torn off and the pore filler can be completely pressed into the cavity comers.
Why should pores on surfaces to be polished filled with
transparent
fillers?
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