Finishing

History

The last 50 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 prehistoric times, 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 2000 years ago. Shellac which is a basic ingredient of French polish, came to Europe from India , where the secretion left by sap-sucking insects on trees has 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 replaced 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 produced yet another range of wood finishes that appear to have the greatest development potential.

Before contemplating which finish to use, all sanding preparation must be complete, nail holes must be filled with stopping (putty), and hands must be clean and free of oil (disposable gloves are a good idea). The type of finishes applied can change the colour of timber, enhance the beauty of a grain and protect the overall surface of a piece.

Preparing for the finish

Raising the grain

If the surface is to be coated with water stain or any other substance likely to raise the grain, damp down the surface with hot water and allow to dry overnight before final sanding . This will remove the raised wood fibres and lead to a smoother finish.

Stopping (putty) is used for filling holes made by nails or natural cracks/splits. There is a variety of stoppings including button shellac, water putty, plastic wood and wax. Shallow dents do not keep putty in place as there is very little key. If a dent cannot be steam ironed out, an alternative, if it must be filled, is to make a few small dents with a marking knife point, which will supply the necessary key to hold the putty The two most common types of putty are water- and oil-based putty and both types come in pre-mixed form. Water-based putty has become more available, can be coloured with water-soluble colours and is suitable under most types of finish.

To apply putty, use a flat-bladed knife and smear it, at a low angle, over the hole so that it is just a little above the surface, let it dry and sand it off with 180 grit along the grain.

Optional processes

Staining

This process is used for four possible reasons:

1. To change the colour of the fibres to produce a more pleasing colour (aesthetics).

2. To make the timber resemble more expensive ones, such as walnut, cedar and teak.

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

4. To match or harmonise the piece with its surrounding furnishings.

Stains, and their application, have advanced a great deal. Moreover, the instructions supplied on stain containers have simplified the process and enabled it to be available to DIY craftspeople.

Types of stains

Stains are classified according to the solvents (bases) that are used in them. There are three main types in general use: oil-based pigment stains, water-based stains and spirit-based stains.

SAFETY: When working with wood stains, solvents and other toxic solutions, gloves should be used and ventilation is necessary.

If using powder stains, cartridge masks may be necessary and it is essential to read all instructions and MSDS before using them. Aniline dyes are carcinogenic and are banned from school use.

Oil-based stains

The main base or solvent in oil-based stains is turpentine (turps); therefore, this is what the brushes must be washed out in later (not water). Shake the container thoroughly as the pigment tends to fall out of suspension. Test the colour first to see if you are happy with it-use a scrap of the same wood or apply it where it will not show.

Apply the stain quickly and evenly with either a brush (watch out for flicking the stain as you go off an edge) or a small pad made from wrapping a little cotton wool/waste tightly into a piece of smooth cotton fabric and dipping into the stain. This will soak up and give you a reservoir, which is slower to apply than a brush but tends to be more even. It is difficult to reach corners with this method and a brush or pointed pad will have to be used. Wait for about seven minutes (see instructions) then vigorously wipe off any excess with a clean rag and leave to dry before applying a finish or the two will start mixing and lifting the stain.

Water-based stains

These dyes have a reputation for rich colours, excellent fade resistance and deep penetration. Results will vary depending on the wood that is used and the light source: natural, fluorescent or incandescent (normal light bulbs), so testing on a spare piece of the same timber is very helpful. Read all instructions on the container as they can vary. Stir the can periodically to maintain a constant colour.

With an oily timber, better stain penetration can be achieved with a weak mix of warm water and washing-up liquid to break down surface tension. Using a soft cloth or fine brush, apply the stain along the grain, allow it to soak in for up to 10 minutes then wipe of the excess stain with a clean cloth . When the area is completely dry, a second coat can be applied for a darker result. Once the timber is totally dry, vigorously rub it with a clean cloth to compress any raised grain and burnish (smooth) the surface ready for the top coat. Wash out brushes with water and a little soap.

Spirit-based stains

Common bases for spirit-based stains include methylated spirits or alcohol. These stains work best on open-pored timbers such as maple, teak, rosewood and red cedar. They are not satisfactory on fine-textured timbers like coachwood and pines. While less penetrating than water stains, spirit­ based stains are also less fade-resistant; however, spirit stains do not generally raise the grain. This type of stain can produce patchy results so an initial thin coat of shellac will reduce absorption but requires two hours for drying before staining. This stain is applied in the same way as the other types and can be lightly sanded after about four hours of drying. A finish is then applied.

Filling

This process involves filling the pores of the grain, which provides a level surface or 'platform' upon which to apply the finish. Although the modern trend in woodworking is to show off the natural open grain of a piece of timber (as if to prove it is not synthetic), filling becomes necessary if one is French polishing antiques or matching existing smooth-filled furniture.

Fillers come in two groups: linseed-oil based and one- or two-pot plastic varieties. Colours are added to match the various colours of timbers, and fillers normally come as a paste that is thinned, as necessary, with mineral turpentine. Drying is by evaporation of the solvent, and a little sinkage may occur. Two days drying time is required, and there may be problems with adhesion of urethane finishes due to the oil content in the filler.

The plastic fillers dry (that is, set and cure) with little shrinking. They are available in the usual timber colour range, dry in as little as six hours and are designed to take nitrocellulose or plastic finishes .

When applying both types of filler, the following steps are involved:

1. Seal the end grain of very porous woods with a wash coat of shellac to control absorption.

2. Mix the filler thoroughly to a smooth paste and thin with turpentine to obtain the correct consistency.

3. Use a thick mix for wood with large pores.

4. Rub the filler freely and vigorously into the pores with a hessian pad. Use a circular action, leaving a skin of filler on the surface to allow for initial shrinkage. Fillers may be applied by brush if desired.

5. When this coat dries (in about 10 minutes), use a clean hessian cloth to wipe off any excess across the grain and check that the pores are filled.

6. Wipe the surface lightly along the grain with a soft , clean cloth to remove cross-grain marks. Take special care to avoid marking the surface with your hands and allow 24 hours for the filler to dry before further finishing. This is necessary because 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.

Bleaching timber

Wood may be bleached for three reasons:

• to lighten its natural colour

• to remove dark spots and streaks

• to obtain a uniform finish on several pieces of timber of the same, or a 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 gums. Always test a small section or off-cut of your work with bleach first.

SAFETY: 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 the area immediately with soap and water.

Types of finish for timber

Water-based finishes

Because of environmental concerns and the simplicity involved with applying them, water­ based finishes are rapidly becoming the most popular type of finish used, especially in paints. Water-based finishes are easy to remove from brushes, there is a low odour, the drying/recoat time is quick, and it is available in a true clear finish which is good for using on pine. However, at the moment, water-based finishes are less durable than oil finishes.

Turpentine-based finishes

Polyurethane is the best known turpentine-based finish. When using polyurethane, be careful as it may have a reaction if used over certain stains or fillers. Generally, turpentine-based finishes have a slight, brown tint when applied. These are one of the most protective finishes available. This level of protection increases with additional layers. The primary disadvantage of varnishes is that they are slow drying, which can allow dust and dirt time to settle and damage the finish.

Nitrocellulose lacquer finish

These are generally used in industry because of their rapid drying time (dries in minutes). However, they require respirators due to the fumes of the solvent, which is also inflammable (check the MSDS before use). Normally, these are applied using a spray gun rather than a brush-unless a slower cure brush type is used.

Methylated spirit finishes

French polish is a typical methylated spirit-based finish. This is one of the few natural resins still in use today. It is quick drying and imparts a red/brown colouration. Shellac, although capable of a superb gloss finish, is not the best finish for table tops, chairs and kitchen cabinets because of the high level of use of these items (and the results of this use such as heat and marking).

Oil finishes

These types of finishes have been used for centuries to treat and preserve wood. Oils are different from most other finishes on the market because they seep into the wood and penetrate the wood's fibres. Because of this, oil finishes cannot be built up to a thick coat and offer less protection but they are easier to apply, which is their primary advantage. Another advantage is that simply wiping on more oil can often repair minor scratches. The most common type is linseed oil, which has an added metallic drying agent that helps the finish dry in a day.

Without this additive, linseed oil can take over a week to dry. Tung oil, unlike linseed oil, does not require drying additives and cures in several days. Scandinavian oil (originally for use on teak) is applied and gives similar protection to the other types of oil finishes .

Blends of oil and varnish are available that give a little more protection than oil alone; however, these are still only meant for one-coat applications.

Wax finishes

There are soft, hard and aerosol waxes. The two waxes that form polish bases are beeswax and carnauba (from the waxy leaves of a palm in Brazil), which have additives to soften and make application easier. Solid waxes can be melted onto wood, which gives a slightly darker effect, or rubbed on dry. When dry, the wood's surface is bu ffed hard and fast to produce a shine. It is recommended that the wood is sealed first and

finely rubbed back to provide a smooth surface for the wax. This provides a simple but effective satin finish for projects.

Liming finishes

This type of finish is good for open -pored timbers like oak, and involves filling the pores with a chalky white (sometimes a very pale lime green) substance, which can be mixed in various mediums (i.e. wax, stain and powder). This gives the project a pale chalky light-coloured appearance.

Types of reflective sheen

There are three types of reflective sheen, and most can be purchased in any of the following: gloss or high gloss, semi-gloss, satin or silk, and flat or matt.

Gloss or high gloss

This produces the shiniest surface and requires care to generate. A gloss, or high gloss finish needs at least two well-prepared coats (the first to seal), which are applied in a dust-free environment and need fine rubbing back, except for the top coat.

Semi-gloss, satin or silk

As the heading suggests, this type of sheen gives the surface a shine that is similar to satin or silk. It is probably the most popular sheen because of its natural look and the ease with which it can be maintained (when compared with a gloss finish). Furniture polish, when applied over this sheen, retains the satin sheen, and it does not show wear and marks as easily as a gloss finish does.

Flat or matt

This type of sheen is used when it is necessary to make the project look as if no finish has been added at all yet the protection is still there. Adding a polish to this type of sheen will make the surface look semi-glossy.

Basic brush finishing

• Ensure that your project, and the area around it, is dust free, and that you have a clean soft brush and jar of clean turps/solvent.

• Spread newspapers over the area but do not leave the job on them to dry as it will stick to them.

• Make a tripod of 12 mm scrap MDF (don't waste real wood on this) with only three, 30 mm nails suitably spaced (four or more nails will allow the project to wobble). Knock the nails right through the MDF just off the edge of the workbench) so that the project can rest on the points rather than on the nail heads. This will make it almost impossible to detect the marks later, as long as you do not push the job across them.

• Plan your working order. Generally, the harder parts and the underneath are the first to be done and the larger easier parts are painted later. Thin edges should be done when the brush is virtually empty of liquid- to prevent a run. Large areas are best completed in strips of about 50-60 mm wide. Each strip should be followed with the next strip overlapping it a little to blend in the brush marks. Always use the brush off an edge (never on it-as this will result in a run). If this does happen, use the empty brush and lightly run it along an arris covering both sides and picking up any loose lacquer.

• If the tin is new, or has not been used for a while, ensure that the lid is firmly on and shake it thoroughly for a minute (with your hands holding the lid on as a precaution). This will mix the contents but may introduce bubbles into the contents. Some say these bubbles are then transferred to the job- but we'll see if this is a problem below.

• Open the chosen tin with an appropriate lever­ opener or flat-bladed screwdriver. Disposable gloves are recommended from this part of the process on. Dip the brush in approximately one­ quarter to one-third of the way up the brush hairs (maximum). Carefully and gently wipe the brush once on the inside of the tin to get rid of the excess. This also helps prevent drips, flicks or runs. Repeat this process on the other side of the brush.

• Work rapidly across the chosen area sweeping diagonally to the left then the right (turning the brush each time). This, effectively fills the grain and covers the surface to be lacquered. Once a large area is completed, don't immediately refill the brush; balance the brush evenly between your finger and thumb, hold it at a lower angle and stroke it softly along the grain. This is called laying off and will even out the surface, get rid of any air bubbles (you might have to repeat two or three times) and allow it to settle without leaving brush marks. Always go off each end and come inwards from any blind corners. The next area must be completed in the same way and, when laying off, blend the lacquer in with the last area, and so on.

• When the project is completed, the tripod can be carefully lifted like a tray and taken into a dust-free room for overnight drying.

• When dry, you can choose between a second coat, waxing or oiling, which can be undertaken

• after a light sanding to flatten dust particles and raised grain, and after dusting off. A final coat is not sanded as this introduces scratches.

When you have finished with the tin, use the brush to sweep round the rim and pick up the excess fluid. Scrape it back into the tin so that the lid will fit on. Note: If the lacquer is allowed to dry in the rim, the lid will not give an airtight seal and the remaining lacquer will start to dry out and form a skin. The lid can then be replaced using a scrap of MDF (or something similar) that fits across the lid and is tapped with a mallet until the lid is firmly fitted.

Very few brushes are thrown out because they are worn out. They are usually discarded because they have been made useless by improper cleaning. Normally, at school, there will be a jar of the relevant solvent to place the brush in. This will keep brushes soft and pliable ready for the next user. If there isn't any jar of solvent available, the brush must be washed (preferably twice) in the specific solvent­ normally stated on the side of the tin. The brush is then washed in soapy water and wrapped in plastic (like cling film).