Wood Finishing Techniques

There is a wide variety of ways in which to finish wood — some more difficult than others; the most commonly used methods are covered here. The standard procedure of finishing is to sand, fill, stain (if required), fill Ale grain, restore the colour (these last two stages apply only to wood with open pores) and seal.

Preparing work

Before attempting any staining, varnishing or polishing, you must ensure the surface of the work is smooth and dust free. If necessary, fill cracks and holes with the appropriate type of filler before carefully rubbing down with flour glasspaper (as already described earlier in the Course). When you are satisfied the work has been adequately prepared, move it to a clean, dry, dust-free environment; be careful to remove or cover all furniture and floor coverings since you may splash and mark them.

Staining

Stains come ready-mixed or as a pigment which you mix with oil, water or spirits. Some pigments are natural products, others are chemical; available in crystal or powder form, most are fairly easy to mix and work with. Once you have decided on a stain, check a compatible finish is available. The simplest way to ensure this to is buy ready-mixed stains and polishes produced by the same manufacturer; otherwise you will have to match up the stains and finishes yourself.

Water stains

These are usually supplied in powder form. Recommended mixing proportions are given in the manufacturer’s instructions and, if these are adhered to, you will obtain a consistent mixture and shade. It is advisable to keep to set quantities when mixing up any stain, especially if it is to be used over a large area; when small amounts are required, the golden rule is to prepare more than you need so you will not have to mix up a second batch which could vary in shade or tone. Before applying the stain, dampen and raise the grain, then sand it down; this will prevent the grain rising later and spoiling the final finish. Water stains are easy to apply and penetrate the timber to a greater depth than other stains; this makes them suitable for furniture or toys which are roughly treated. Also water stains will not cause a chemical reaction with the finish. Oil stains The base of these stains is naptha oil, a coal/petroleum by-product. They must not be used if a polyurethane finish is to follow, since a reaction will set in between the stain and the finish. Oil stains come ready-nixed as well as in pigment form.

Spirit stains

These stains are less popular than the others because their volatile nature and quick-drying properties make them difficult to work with. They are mostly based on methylated spirit and, like oil stains, contain aniline dyes as the colouring agent; this means they should not be used in conjunction with a polyurethane finish. Speed of application is essential; it is inadvisable to cover large areas with this type of stain since some parts will dry before others, leading to patchiness.

Using stains

The basic tool for applying stain is a brush and, as a general guide, the larger the surface to be treated, the larger the brush; awkward areas such as mouldings should be worked with a fitch brush. On large areas, such as table tops, use a dry sponge or lint-free rag so an even covering can be applied quickly.

Before staining, remove all fittings from the work and, if possible, place the surface to be stained in a horizontal position. Start applying the stain about 25mm in from one end and work towards the other; recommence 25mm (1 in) in from that end, overlapping the stained edge to cover the unstained area. Use long, even strokes and always work with the grain.

You can rub the stain into the wood with a clean cloth to guard against splashing; this is most important when one area of the work is to be stained a different colour from the rest. To minimize absorption, lightly wipe the surface with a clean cloth, maintaining even pressure. If you want a darker colour, apply a second coat. Always treat the end grain with a half-strength mixture — you will have to thin ready-mix stains — so it will end up the same colour as the rest.

Never leave part of the job unfinished, since it will be difficult to obtain the exact shade again. Check no area has been missed and set the work aside for several hours to dry.

Filling the grain

Grain fillers are not required with softwoods, most man-made boards and ramin, but they are essential for a good finish with open grain woods like oak and mahogany. There is a large variety of fillers, but the two most commonly used are plaster of Paris and paste. Colouring pigments and water, spirits or oil (depending on the type of stain used) are added to produce a creamy slurry which is worked over the wood with a brush or rag. The excess is wiped off with a rag and the surface lightly rubbed down with flour glasspaper.

Plaster of Paris

Since this sets very rapidly, large areas should not be attempted at one time and any excess must be removed as quickly as possible with a coarse rag; an offcut of tapered dowel can be used to remove filler from recesses. After the plaster has set hard and been rubbed down, the full colour of the stain can be restored by rubbing in raw linseed oil which removes any remaining plaster. If you are going to apply French polish to the work. You must fix the stain with two or three coats of French polish before applying the plaster of Paris filler. Don’t use plaster of Paris with polyurethane since it will cause a chemical reaction.

Paste

This is available ready-mixed in a variety of wood shades. Follow the manufacturer’s instructions — they do not differ markedly from those for plaster of Paris — and leave for 24 hours to harden.

Finishing

When deciding on a finishing method for wood. Always consider the use to which it will be put. Since some finishes are more suitable for a particular purpose than others. Also, if you have had little experience, it would be best to choose one of the simpler finishes to achieve good results, rather than attempt something complex and spoil the work.

Using French polish

For many years French polish was the prime method of furniture finishing; if applied properly. It can give superb results. It does, however, take much patient and painstaking practice to acquire the necessary skills and techniques. French polish is difficult to apply to carved or moulded surfaces because for a perfect finish the surface must be as smooth and as flat as possible. The finish is not resistant to heat, water or scratches.

French polish is obtainable ready-mixed in a large variety of colours and shades. Alternatively you can buy shellac — the basic ingredient — and dissolve it in methylated spirit; use about one kilogram of shellac to four and a half litres of methylated spirit (or 2 ilb shellac to lgal methylated spirit). Apply the liquid polish with a pear-shaped rubber made of rags and wadding; use light, even strokes in a series of up and down, circular and figure of eight motions. Alternate these strokes until a high, almost mirror-like gloss is achieved. As more polish is applied, the methylated spirit evaporates and it becomes progressively difficult to keep the rubber moving across the tacky surface. You can overcome this by dropping the smallest amount of linseed oil onto the rubber; but be careful to use only the barest minimum otherwise the oil will smear and mark the surface.

Using polyurethane

Polyurethane is the most popular and durable of finishes and has several advantages over French polish. It is tough and resistant to heat, water and scratching; it can be applied quickly and easily with a brush and it sets to a good finish in gloss, matt or eggshell. Polyurethane must not be used with any stain or filler which has been mixed with linseed oil, but it can be used over many finishes provided they have hardened. You must remove all traces of wax and oil with turpentine before application. If applying more than one coat, leave each to dry thoroughly before applying the next: buff alternate coats with the finest grade of steel wool, ensuring the penultimate coat is buffed (but not the final one).

Using rubbing varnish

This varnish dries exceptionally hard and is easy to apply. Several finishes can be achieved, but the natural appearance is gloss; you can obtain a matt or eggshell effect by rubbing down with the finest steel wool combined with wax polish. In a warm room, apply the varnish carefully with a brush: leave it to harden for at least 24 hours before rubbing down with silicon carbide paper and soapy water and applying a second coat. After two days. Rub down the final coat with rottenstone — a powdered abrasive which you work over the surface with a damp pad. Wipe the surface clean and polish it with liquid car polish; don’t use an ordinary wax.

Using oil

Oil polishing is a simple procedure, but it does require a lot of energy. Oil is rubbed into the grain with a soft rag over varying lengths of time. Depending on the oil used.

Linseed oil

This should’ be applied at regular intervals over a five to six week period so a water-resistant finish is built up.

Teak oil

Although primarily intended for teak, this can be used on other oily woods and is particularly suitable for furniture which is to be used outside. Teak oil contains drying agents which speed up application and also improve its resistance to heat in comparison with linseed oil. It will never polish up to a very high gloss finish, so is unsuitable for good quality furniture. Destroy the cloths after use since they will be highly inflammable.

Danish oil

This can be used on all types of wood, is easy to apply and is suitable for both interior and exterior use, although periodic re-oiling will be necessary if used outside.

Olive oil

This should be used as a finish on work which will come into contact with food.

Using wax

Originally the basic ingredient of wax polish was beeswax, but some modern polishes contain silicone since this is more resistant to marking. Wax polish can be applied effectively to most wood and, with age, can result in a fine stain gloss. It can also be used successfully over other finishes to give added lustre. Rub it into the grain with a lint-free rag.

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