Preparing And Finishing Wood : A Full GUIDE
There could be three reasons why you want to give the wood some finishing treatment. You may want to seal the new wood, touch up an existing finish, or you may want to strip off an existing finish and start again, perhaps to give the wood a different colour.
But before you apply any wax, varnish, oil, colour, or paint, spend some time on preparation. You’ll end up with a smoother, more effective, and more professional finish.
Preparing Wood to be Treated
Before you start any work on colouring or finishing wood, you must make certain that all the surfaces are properly prepared, to ensure that you’re able to apply any treatment effectively, and end up with a professional finish.
But how you’ll prepare will depend on the condition of the existing finishes. You’ll treat new wood quite differently from old wood that may be cracked, blistered, or flaking.
What’s the Surface Like?
All wood that has not been treated from, flat-pack furniture to floorboards, is prepared in the same way:
- sand it lightly with 180 grit/240 grit sandpaper
- remove all dust with a vacuum cleaner
- wipe with a cloth moistened with white spirit to remove any traces of grease.
The wood is now ready for one of these treatments:
- varnish, oil, or wax
- colour with wood dye or stain followed by varnish, oil, or wax
- colour and finish with an all-in-one product.
If the wood has a lot of knots or is very resinous (oozing sap), the affected areas should first be sealed with knotting solution, which can be brown or colourless. Use a colourless version if you want to apply a translucent finish on top.
Applying knotting solution will affect the wood’s ability to absorb the wood dye, and it may be incompatible with some finishes, so make sure you check the manufacturer’s recommendations.
Existing Finishes in Good Condition
A finish in good condition is fulfilling its protective role and is best left intact if possible. If you want to change the appearance, consider these options.
- Paint can be overpainted with a different colour.
- Most varnishes can be painted over (check in an inconspicuous place to make sure there is no incompatibility).
- Colourless and light varnishes can be recoated with a coloured varnish. You can apply acrylic varnish over polyurethane varnish and vice versa as long as the old finish has completely cured – two weeks for acrylic varnish, one month for polyurethane varnish. Just sand lightly to provide a good surface for the new finish and remove dust and grease as above.
- With French polish, oil, and wax finishes you can only apply more of the same. If you want to alter the appearance radically, these finishes must be stripped back to the bare wood.
Existing Finishes in Poor Condition
- Cracked, blistered, or flaking finishes are usually found on exterior woodwork and garden furniture.
Remove all the loose finish. Use a shavehook and work with the grain, pulling the tool towards you.
- Rub down with sandpaper.
- Use dampened wet-and-dry sandpaper if the paint might contain lead.
- Leave any paint or varnish that is hard to remove unless there is a marked step between the surface of the sound paint and the exposed wood.
- If there is more paint than wood visible, the filling may be a better option than stripping.
- Oil and wax finishes can be revitalized with several new applications of the same finish.
Methods of Stripping
This is quick and a good first option if you are tackling a large or fixed item, like a staircase, which you can finish stripping with a chemical stripper.
Heat the paint with a hot-air gun until it softens enough to be removed with a scraper. The melted paint will still be hot, so collect it in an old metal, rather than plastic, container. Heat stripping is not suitable, however, for paints containing lead.
Hot-air guns are preferable to blowtorches, which can easily burn the wood. Guns have interchangeable nozzles to direct the heat to where it is needed, including one designed to protect the glass.
This is slower but useful for small or fiddly projects. It is the best option for paint that might contain lead and can also be used on metal and glass. Some lacquers are not affected by chemical strippers, and you may need to use cellulose thinners to remove them.
For good results don’t skimp on the amount of product used, or the time allowed for it to take effect. Remove most of the softened paint with a scraper or shavehook, then use coarse steel wool dipped in the stripper to remove ingrained paint or varnish. Follow the manufacturer’s instructions for neutralizing the stripped surface.
Methylated spirit removes shellac (a hard, glassy varnish) and French polish (the traditional polish found on many older pieces of furniture). To test the finish, rub it with fine steel wool moistened with methylated spirit.
If it softens to a treacle-like consistency, the finish can be removed, using steel wool, shavehooks, and copious quantities of methylated spirit. It is slower and messier than other methods, but kind to the wood underneath.
Restoring a French-polished surface is a professional job, although amateurs can achieve passable results on small areas like picture frames or chair legs. Never strip the finish from a potentially valuable item, as you will drastically reduce its worth.
White spirit and linseed oil (mixed in a ratio of 3:1) remove a build-up of dirty wax to prepare for a new wax finish. Apply it with a cloth or steel wool and rub vigorously. Finish by wiping over it with white spirit and leaving to dry. If you want to apply a different type of finish, the surface will need to be thoroughly sanded.
When using a chemical stripper, always wear protective clothing and work in a well-ventilated place.
In this method, the item is immersed in a tank of caustic solution. It is a tempting option for large or very intricate pieces, but there are serious downsides.
- Caustic solutions dissolve old-fashioned glues, causing joints to loosen and veneers to lift.
- Some woods, such as oak, are badly discoloured by caustic stripping.
Some firms offer a gentler, non-caustic stripping service, but this process is more expensive.
Prepare The Wood For Coloring
Some timbers look fine with just a clear oil, varnish, or waxed finish. Others don’t – they may have shade variations that need to be evened out or the wood may not be the colour you want.
If you want to enhance or change the colour of, say, a piece of furniture, a door, or floorboards, without obscuring the pattern of the grain, there’s a wide range of products to choose from, but there are important differences between stains, dyes, and varnishes.
Choosing The Right Colour
- Check the natural colour: wipe the bare wood with a cloth dampened with water or white spirit to give an idea of what the wood would look like with a colourless oil or varnish.
- Even treated woods will darken with age, so choose a colour one shade lighter than the final colour you want.
- Colour charts and samples are for guidance. The only accurate test for colour is to try it on an identical piece of timber – either an offcut or an inconspicuous patch, for example under a table top.
- Take into account the effect of the finish on the colour. Most polyurethane varnishes add a yellow tinge.
- Even the best-quality colourants can vary from batch to batch. Buy sufficient quantities to finish the job. If you need more than one tin, mix them in a bucket before you start.
- If dyeing, check that the product is compatible with your chosen finish. Play safe by using products from the same manufacturer and the same range – a quick-drying varnish over a quick-drying dye, for example.
Choosing Between Dyes & Stains
The key difference between wood dyes and wood stains is that dyes do not protect the wood. Dyes need a finishing coat of varnish, oil, or wax.
Overall, a dye gives more control over the depth of colour and, properly finished, will be more permanent.
Wood stains give both colour and protection. And unlike tinted varnishes, which will eventually blister, crack, or flake, wood stains just wear off, making them much easier to recoat or touch up at a future date.
They are available for interior and exterior use, vary in density (some are nearly opaque) and come in a fairly limited range of wood colours.
Spirit-Soluble Wood Dyes
These are available in a variety of timber colours. They penetrate thoroughly, giving a good finish on close-grained hardwoods.
You need to wait at least six hours for the dye to absorb before applying the finish.
These come in a wide range of shades. While timber tones are ‘classics,’ the availability of other shades tends to be driven by fashion. They may even be marketed under a different name, such as a wash.
Shades can be discontinued fairly quickly – another reason for buying enough product to finish the job. Quick-drying dyes are easy to use: they are touch-dry in about 20 minutes and ready for a second coat or the finish after an hour or so.
Concentrated dyes, both water- and spirit-based, give you the flexibility to create your shades.
All in One Waxes & Varnishes
All-in-one tinted varnishes and waxes colour and seal in one operation. The colour sits on the surface of the wood, rather than being absorbed. This means you have less control over the depth of colour achieved.
Any damage on the surface or imperfections – like runs – will be more conspicuous, and the colour wears off with the finish.
These products can be used to alter an existing finish, but check on an inconspicuous place that the old and new finishes are compatible.
- Remove the existing finish using one of the methods described here. If the old finish has deeply penetrated the wood, it might affect the absorption of a dye or stain – seal the surface with clear varnish and then build up the colour with tinted varnish.
- Lightly sand stripped or new wood with medium-fine paper (a very fine grade could prevent even absorption). Sand in the direction of the grain of the wood – any scratches across the grain will be conspicuous after colouring.
- Use steel wool rather than sandpaper on shaped surfaces like chair spindles.
- Water-based dyes can raise the wood red cedar grain, leaving a rough finish when dry. To prevent this, wipe the wood down with a wet rag. Leave to dry, then sand smooth.
- Remove all dust, using a vacuum cleaner attachment to reach awkward crevices.
- On flat surfaces use a lint-free cloth – for instance, an old T-shirt folded to make a pad – or, for quick-drying dyes, a paint pad or sponge applicator. Use a brush to carry colourant into corners, mouldings, and carvings.
- Shake the colourant and decant it into an old dish big enough to take your pad or brush.
- Work quickly with the grain of the wood, keeping edges wet and blending them as you go.
- When dyeing or staining floorboards, use a 100 mm (4 in) brush and cover two or three planks at a time, stopping at edges and ends.
- Allow the colourant to penetrate for the recommended time, then wipe off the excess, working with the grain.
- Leave until completely dry, then buff with a coarse cloth to remove surplus. If the effect is patchy, or the colour is not deep enough, apply a second coat.
Transparent protective finishes – oil, wax, or varnish – are designed to allow the wood grain to show through. They can be used on bare wood or over compatible dyes but will only be effective if the surface has been properly prepared.
The amount of protection varies from product to product: take into account how much wear the wood will receive, choose a finish that is tough enough to cope, and follow the manufacturer’s instructions.
Choosing Which Varnish to Use
Polyurethane varnishes are the most commonly used modern varnishes. These varnishes are very hard-wearing and can be used to coat timber inside and outside the home. Acrylic varnishes are nonetheless becoming increasingly popular.
Advances in manufacturing methods mean that these, too, are suitable for interior and exterior use, and even for hard-working surfaces like floors.
Although some people maintain that polyurethane still has the edge in terms of durability and glossiness, others are won over by acrylic’s user-friendly qualities.
Acrylic varnish does not have the unpleasant odor associated with polyurethane, it is non-toxic, it dries more quickly, and brushes used for acrylic varnishing can be cleaned with water.
Achieving a Perfect Finish
- If you want a smooth, gloss finish on an open-grained wood such as oak, ash, or elm, use a grain filler. Choose a filler that matches the wood colour, adding a few drops of dye if necessary. Apply one coat of varnish first, and allow to dry. Brush the filler into the wood, working with and across the grain. Remove the excess with a coarse cloth. Lightly sand with the grain when fully dry.
- Before using polyurethane varnish, seal the surface with one coat of varnish diluted by 10% with white spirit. Use a soft cloth pad to work the varnish into the grain, and a brush to get into corners and mouldings.
- Before applying a wax finish, seal the surface with a diluted coat of varnish.
- All brushes must be clean, as any old paint clinging to the bristles might spoil the finish.
- Tap excess varnish from the brush. Make sure you don’t squeeze or scrape the brush against the tin, as this can create air bubbles in the varnish.
- Whichever finish you are using, allow each coat to dry completely and rub down with fine sandpaper or wet-and-dry paper before applying the next coat.
- If, before the last coat of any finish, there are imperfections – such as dust, stray bristles, or runs – remove these with wet-and-dry paper when the surface is completely dry, dust, then apply the final coat.
- If dust settles on the surface you have been working on while the last coat is still wet, let it dry, then use the back of some sandpaper, wrapped the wrong way around a sanding block, to clean off the dust without scratching the surface.