WOOD FINISHING

The last thirty years have seen revolutionary advances in the development of materials and techniques for wood finishing, particularly for furniture and cabinet work. However, decorative and protective coatings have a very long history, probably dating back to prehistory, when people used animal fat to preserve and strengthen wooden weapons and implements. The ancient Egyptians made water-thinned paints from casein and egg­white, and varnishes from fossilised resins. Chinese and Japanese craftspeople were using lacquer made from tree sap more than two thousand years ago; and shellac, which is a basic ingredient of french polish,' came to Europe from India, where the secretion left by sap-sucking insects on trees had been gathered and used as a wood preservative since before the time of Julius Caesar.

Today we use synthetic lacquers, which, in industry at least, have largely supplanted french polish because of their ease of application and their greater resistance to heat, moisture and abrasion. Since the end of World War II accelerated research in the field of plastics has brought forward still another range of wood finishes that appear to have the greatest development potential.

SURFACE PREPARATION Patience and skill in preparing the surface are essential to producing a good finish, especially where a high polish is required, for this will reflect the light and accentuate imperfections' such as plane marks, sanding scratches, bruises and rounded edges.

Having made sure that all faces and edges are true, carry out the following steps:

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1. Clean off with a smoothing plane sharpened super-keen and set to produce an extremely fine shaving. Set the cap iron very close to the cutting edge, especially for timber with knots or interlocked grain (Fig. 16.1). Plane the entire surface in the direction that allows maximum working with the grain, then reverse the work and plane areas of torn grain (Fig. 16.2).

Note: Before cleaning off machine-planed timber, raise the grain by damping with hot water. This eliminates the risk of fibres that have been compressed during machining expanding when finishing materials are applied.

Fig. 16.1 Cutting edge of smoothing plane should be straight with corners slightly rounded on oilstone; note close setting of cap iron

Fig. 16.2 Cleaning off with smoothing plane

WOOD FINISHING

Fig. 16.3 Using hand scraper after planing

2. Remove plane marks and tears, particularly around knots, by scraping the entire surface with a hand scraper or cabinet scraper (Fig. 16.3).

3. If the surface is to be coated with water stain, french polish, or any other material likely to raise the grain, damp down with hot water and allow to dry overnight before sanding.

4. Sand along the grain with glasspaper or garnet paper, working over the surface several times (Fig. 16.4). For flat or singly curved surfaces, the abrasive paper should be wrapped around a cork rubbing block to prevent rounding of edges and non-removal of high spots. For most cabinet timbers, begin with 80-100 grade paper, follow with 1'20 grade and finish with 180 grade. The end grain requires a much coarser abrasive than the face or edge grain. Very fine papers, such

Fig. 16.4 Cork block supports abrasive paper for sanding flat surface; sand with grain

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as 240 grade upward, tend to burnish rather than sand.

5. Fill all holes or break in the surface with suitable 'stopping' (see below).

Stoppings Button shellac This is an excellent non-shrink stopping, but is dark in colour and therefore suitable only for use on dark-stained or dark-coloured timbers. It can be run into holes by melting with a piece of hot metal or an open flame.

Lacquer putty This is a comparatively.quick-setting stopping available in a wide rang~ of colours. Press it into the hole with a knife tip, leaving a little standing above the surface to allow for shrinkage. Leave to dry for at least 30 minutes before sanding level, but if the timber is to be oil finished, sand off immediately with medium-grade paper to remove all putty stain from around the hole.

Water putty A casein-based powder, this is mixed to a paste with water and applied with a knife tip. It can be coloured with water-soluble colours and is suitable under most types of finish, but casein may stain some timbers.

Plastic wood This is available in natural or coloured tints. It is plastic and easy to use, but must be built up well above the surface to allow for shrinkage and tends to discolour fibres if not used with care.

Beaumontage This is a home-made non-shrink stopping comprising equal parts of beeswax and crushed resin melted together with a few flakes of shellac, mixed thoroughly and poured into a mould to form a stick. Apply by remelting with a piece of hot metal or an open flame and pouring into the hole. Allow to cool, then chisel off and sand. Its use is limited by its dark colour and the fact that stains will not take over it.

220

Oil putty This may be used under oil finishes, but is not suitable for plastics, since it tends to shrink and discolour fibres around the hole.

Selecting colour of stopping 1. For natural timber finish, match stopping to

colour presented by timber when dabbed with mineral turpentine.

2. For stained or filled timber finish, choose stopping with same tint as stain or filler, but of a darker shade.

Some common faults in timber surfaces and how to cure them Machine marks Draw a scraper across the line of marks. Finish with glasspaper or garnet paper, working with the gram.

Cross-sanding marks Use a scraper and glasspaper or garnet paper with the grain.

Bruises For shallow indentation, apply hot water directly to the bruise, or by a hot iron to damp cloth laid on the bruise. These methods may damage veneered or plywood surfaces. Another method is to pour a little methylated spirit into the bruise and ignite.

Blisters in veneer Slit the blister and insert glue, then choose one of the following methods:

1. Rub with a hammer face until the glue holds. 2. For a small blister, nip the point off a brad

and drive it almost home. When the glue has set, extract the brad or drive it below the surface.

Surplus glue Remove large deposits with a chisel. Clean off with a scraper and glasspaper or garnet paper, working with the grain. Some glues respond to sponging

WOODWORKING PART ONE

with warm water, but care must be taken not to soften the glue under the veneer as it may lift.

Dirt and grease marks Wash dirt with water, grease with benzine. Note danger that these methods may convert dirt or grease into a weak stain that is difficult to remove. Finish with glasspaper or garnet paper, working with the grain.

Water marks Dampen with water and use abrasive paper with the grain.

Pencil marks Rub lightly along the grain with abrasive paper. If the mark is indented, treat as a bruise (see above) and also wipe off pencil discolouration with a damp cloth.

STAINING TIMBER Woodwork is stained for one or all of four reasons:

1. to change the colour of the fibres to a more agreeable one;

2. to maintain the natural beauty of the wood and throw into relief the different degrees of light and shade presented by the grain.

3. to create an even tone of colour over the whole piece, especially when different varieties of the same timber have been used;

4. to match or harmonise the piece with other furnishings.

Much manufactured furniture is made from cheaper timbers and then stained to resemble more expensive ones, such as cedar and teak. Industry also uses shading stains to colour timbers such as klinki pine, kauri and coachwood that are difficult to stain by normal methods. Shading stains are generally mixed with sanding sealers, shellac solutions, lacquers or other finishing materials and sprayed on the filled surface.

To achieve uniformity of colour in stained woodwork, follow these procedures:

1. Where the piece is made from more than one kind of wood:

WOOD FINISHING

(a)

t,' If III fOT(J Cl r...J~0 Ci 0 rfD7V:V-~fi51Ol.::-:r IJJUlJ5 - C ~ J Q COO 0 alU[otOfQ 0 () 000 ulO[DJO]O- 0 C

- @ 0 0 oln 0 CJ U,U 0 a 0,0 0 0 0 OIOlOlult!, c I I! a , 0 J'G \1 0 Ol~ u a 0 C' () (J U u O}Cl}OlO1O; c

If] 0 00 u 0 D f2lO:, 0 0 cl OIDJa O)OIOIoIOJO d C ~lr a 00 [] i) 0'00 tl 0 010f01O '; u OlDJ01(J'ol&~

- .- - 0 <-> D Dlntn oio a ') (b)

Fig. 16.5 Sections showing cell structure of (a) hardwood or pored timber and (b) softwood or non-pored timber

(a) coat the more absorbent timber last, after weakening the stain by adding more solvent; or, preferably,

(b) coat the more absorbent timber first with with free stain solvent and while this is still wet apply stain to the whole piece.

End grain and rotary-cut veneers should always be treated in one or other of these ways.

2. Where the piece is made from the same kind of timber throughout, treat with a second coat any areas that show a lighter colour after the first staining.

The structure of the timber being finished shouldbe carefully considered (Fig. 16.5). Even when carefully planed and sanded, most pored timbers have minute surface cavities requiring filling before fmishing. Non-pored timbers present a relatively smooth 'platform' for finishing and require sealing only. Schematic sections similar to Figure 16.5 accompany the application instructions for various finishing materials in the following sections; they emphasise the importance of understanding timber structure in determining correct procedures.

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Fig. 16.6 Common types of stains: spirit, pigment and water

Types of stain Stains are classified according to the solvents employed. There are three main types in general use: pigment stains, water stains and spirit stains (Fig. 16.6).

How to apply pigment stains Pigment stains, the simplest to apply, consist of oil-soluble dyes dissolved in turpentine, naphtha or similar oil, and are available in a wide range of colours that can be blended to produce any required tone. Pigment stains do not raise the grain in timber, but they are less penetrating than water or spirit stains and, hence, tend to fade. They are slow drying, may 'bleed' through some fmishes, and cause adhesion problems with certain plastic finishes, particularly the polyurethanes.

1. Having prepared and 'stopped' the timber surface, select the stain colour desired and lighten, if necessary, by adding turpentine. Test the colour on a scrap of the same wood as the work.

2. Coat all exposed end grain with linseed oil to control absorption and prevent the end grain becoming darker than other surfaces.

3. Apply the stain evenly and quickly to the whole surface, using a wadding pad or brush.

4. Wipe off excess stain along the grain with a clean cloth.

5. Allow to dry overnight, then seal with a wash coat of shellac. This prevents fillers, subsequently applied, from absorbing the stain, and also reduces the risk of the stain 'bleeding' through the finish.

222 WOODWORKING PART ONE

How to apply water stains which can be modified to an infinite number of shades by varying the quantity of water. The dye powder must be thoroughly dissolved in order to avoid flecking of the surface and unevenness of tone. A disadvantage is that water stains raise the grain of timber, but this is counteracted by pretreatment (see below).

Made by dissolving water-soluble dyes, such as aniline dyes, in hot water, these stains penetrate well and have excellent fade-resistance, bringing out the full beauty of the wood. They are inexpensive and can be obtained in many colours,

1. Prepare the surface and apply stopping if necessary. Raise the grain by spong­ing lightly with warm water (Fig. 16.7 (a)).

2. Allow from two to three hours drying time before sanding lightly along the grain with 0 or 00 grade paper (Fig. 16.7 (b)).

HARDWOOD

(b) 3. Mix the colour and shade of ~~Jes:Of~:;:~:tains are suitable forstaining both hardwoods

(c)

Fig. 16.7 ApplyinfJ. a water stain

stain desired (Fig. 16.7 (c)). To avoid matching problems, always mix more stain than necessary. Apply two light coats in preference to one dark

- coat.

4. Apply the stain liberally with a brush or cloth, working quickly over each surface (Fig. 16.7 (d)). Work the stain into the wood by sanding along the grain with 400 grit wet-or­dry abrasive paper.

5. While the surface is still wet, wipe off excess stain with a soft, clean cloth in the direction of the grain (Fig. 16.7 (e)). Allow 24 hours to dry before lightly sanding with 00 grade paper.

SOFTWOOD

Fig. 16.8 Schematic section of water stain application

WOOD FINISHING 223

How to apply spirit stains red cedar, but are not satisfactory for fine-textured timbers like coachwood and the radiata, Parana, kauri and klinki pines. Less penetrating than water stains, they are also less fade-resistant, and sanding should be kept to a minimum.

Dyes soluble in methylated spirits or alcohol, most of them derived from coal tar, are used in spirit stains. They are suitable for most of the open­pored timbers,,-such as maple, teak, rosewood and

1. Prepare the surface and apply stopping if necessary. Spirit stains do not generally

. th . b Ii \OlJTii7l IC.\,(\"I,I-D IT),\3 ralse e gram, ut test or

(a)

(b)

(c)

(d)

Fig. 16.9 Applying a spirit stain

this and, if necessary, raise the grain and sand before staining (see 'How to apply water stains' step 1).

2. Select the colour and lighten if necessary by adding stain reducer or methylated spirits. Colours may be mixed to give the desired shade. Test the colour on a scrap of the same wood as the work (Fig. 16.9 (b».

3. Coat the end grain and turned work with a wash coat of shellac (100 g to 1 L of methylated spirits) to prevent excessive absorption of stain and the resulting contrast in colour (Fig. 16.9 (c». Allow two hours drying time.

4. Apply the stain liberally with brush or rag (Fig. 16.9 (d». Work quickly with the grain, covering a small area at a time so 'that step 5 can be carried out before the stain dries.

5. Wipe off excess stain along the grain with a soft cloth while the surface is still wet (Fig. 16.9 (e». Allow from two to four hours to dry, then sand down lightly with 00 grade paper before proceeding with further finishing.

Fig. 16.10 Schematic section of spirit stain application

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FllUNG TIMBER The purpose of filling is to provide a level surface of 'platform' upon which to apply the finish. Most wood used to make furniture and interior fittings is of the open-pored type and, if the pores are not filled, wasteful quantities of lacquer, polish or varnish are needed to produce the required gloss and there is risk of shrinkage in the finishing material. Non-pored or fme-textured woods, such as Parana pine and coachwood, do not need filling, but a sealing coat of sanding sealer or white shellac should be applied to form a barrier between the timber and finish and prevent shrinkage of the finish coats.

Wood fillers may be divided into two groups: linseed-oil based fillers, and curing one-pot and two-pot plastic fillers (Fig. 16.11).

Linseed-oil based filler is made from silex (a ground silica) mixed with a drier (gold size or terebene), a solvent (thinner) and the vehicle (linseed oil). Colours in oil are added to give the desired tint. Proprietary brands are available with these ingredients ready mixed to a paste that requires only thinning with turpentine and stirring to the right consistency. Colours available include maple, cedar, teak and other timber tonings as well as a neutral filler, and any desired colour or shade can be produced by intermixing. Since these fillers dry by evaporation of the solvent followed by oxidation of the oil, some sinkage does take place, and from 12 to 48 hours should elapse

WOODWORKING PART ONE

Fig. 16.11 Types of wood fillers: (from left) oil-based paste filler, one-pot plastic filler, two-part plastic filler-sealer, two-pot clear filler for inlay or marquetry

before a sealer is applied. The oil content may cause adhesion problems with some plastic finishes, particularly the urethanes.

Curing one-pot and two-pot plastic fillers are generally based on polyurethane or polyester plastics, which dry (set and cure) by chemical change and, hence, are subject to very little sinkage. Available in a range of timber colours and neutral, as well as in a transparent form especially suitable for inlaid and marquetry work where it is important for the grain to show through clearly, these fillers are designed for use under nitrocellulose lacquers and polyurethane or polyester finish coats. Normal drying time is only from four to six hours, and they can be force-dried in one hour at from 27 ° to 32°C.

How to apply wood fillers

(a)

1. Select a suitable type and colour of filler and read the manufacturer's directions carefully. Prepare the sur­face, apply stopping if necessary and stain if desired (Fig. 16.12 (a)). Non-pored woods such as pine do not require filling.

SOFTWOOD

WOOD FINISHING

(b)

(c)

(d)

(e)

Fig. 16.12 Applying wood filler

2. Seal the end grain or very porous woods with a wash coat of shellac to control absorption (Fig. 16.12 (b». Allow two hours drying time before sanding with 00 grade paper.

3. Mix the filler thoroughly to a smooth paste and thin with turpentine to obtain the correct consistency. Use a thick mix for wood with large pores.

4. Rub the filler freely and vigorously into the pores with a hessian pad (Fig. 16.12 (c». Use a circular action, leaving a skin of filler on the surface to allow for initial sinkage. Fillers may be applied by brush if desired. Leave the filler for from five to ten minutes to 'flat off. Do not allow it to dry too long or steps 5 and 6 will be difficult.

5. Wipe off excess across the grain with a clean hessian cloth (Fig. 16.12(d». Check to see that all pores have been filled.

6. Wipe the surface lightly along the grain with a soft, clean cloth to remove cross marks (Fig. 16.12 (e». Take special care to avoid marking the surface with your hands. Allow 24 hours to dry before further finishing, as the filler leaves a sealing skin that should not be broken.

Note: A shellac solution may be used as a sealer over oil-based fillers.

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HARDWOOD

Fig. 16.13 Schematic section of wood filler application

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BLEACHING TIMBER Wood may be bleached for three reasons:

1. to lighten the natural colour; 2. to remove dark spots and streaks; 3. to obtain a uniform finish on several pieces

of timber of the same or different species, whose natural colours vary.

Because of their structure and colouring, maple, ash, oak, mahogany and walnut bleach more readily than pine, rosewood and gum.

Always test-bleach a small section or offcut of your work.

Bleaches are composed of strong chemicals and must be used with the utmost care. Protect your eyes, wear rubber gloves, and work in a well-ventilated area. If bleach contacts your skin, wash immediately with soap and water.

Bleaching solutions should be prepared in glass containers and may be applied with a brush, cotton cloth or rubber sponge.

Having sanded the timber smooth and clean, proceed with one of the two following methods:

Ammonia and hydrogen peroxide 1. Apply 20 % concentrated ammonia solution. 2. While this is still wet, apply 30 % hydrogen

peroxide (100 volume). 3. Allow the work to stand for from 36 to

48 hours. 4. Repeat steps 1-3 until the required degree

of bleaching is obtained. 5. Coat with thin white shellac or clear lacquer

before filling.

Oxalic acid and sodium hypochlorite 1. Dissolve 80 g oxalic acid crystals in 1 L hot

water and allow to cool. 2. Apply this solution liberally and allow to

soak in.

WOODWORKING PART ONE

3. Apply sodium hypochlorite (household laundry bleach) and allow to dry.

4. Neutralise the bleach by washing the surface with a solution of 26 g borax in 1 L water.

5. Allow to dry for at least 48 hours, then lightly sand the surface if the grain has been raised. Avoid unnecessary sanding, as the bleach affects only a very thin surface layer.

TYPES OF FINISHING MATERIALS Finish coatings can be classified in three groups:

1. those that dry by evaporation of the solvents; 2. those that dry by oxidation of oils following

evaporation of the solvents; 3. those that dry by chemical change.

Finishes that dry by evaporation French polish is a spirit varnish made from shellac dissolved in methylated spirits. Use orange shellac for medium-coloured woods, white shellac for light. woods and multicoloured surfaces such as inlay and marquetry, and button shellac for dark timbers like cedar and rosewood. The proportion of 1.5 kg of shellac to 4 L of spirit makes a good general mixture.

French polish is easy to apply and dries rapidly to a fairly hard finish, but is not waterproof. Thinned french polish of about half the usual strength makes a good sealer under other types of finish, controlling absorption and sealing gum veins, knots and similar blemishes.

Lacquer is now usually formulated on a nitrocellulose base. To this are added natural and synthetic resins, for durability, gloss and adhesion;