Main page Læs denne side på dansk

Finishinhg of wood

I have the impression that there are as many opinions about finishing of wood as there are people working with wood. Especially, it varies much how people like the wood surface to look and feel. Some like a shiny surface as obtained with wax or lacquer, others like a smooth silk like oil finish and some like a more rough surface.

The robustness of the different types of finish also varies a lot. Wax can be delicate, oil may be washed away after repeated cleanings with soap, lacquer can peel of if it get a small crack followed by humidity.
    Therefore, you can't say anything simple about finishing. I mostly use virgin linseed oil or simply food oil from the cupboard in the kitchen. Sometimes I use Rustin's 'Danish Oil', 'teak oil' (which I find useful on pine wood), carnauba wax or lacquer.

You can here jump directly to:
Sanding,   Pure oils,   Mixed oils,   Waxes,   Lacquer,   Preventing sun colouring,   Bleaching,   Ammonia,   Scorching,   Sandblasting,   Wire brushinging,   Linseed oil/wood tar for outdoor use


It isn't a tall story: Rags with oil may self-ignite!!!

But it isn't necessary to burn rags right after use as it is written in some books. I put my used oil rags in an old metal coffee canister without lid so that it doesn't make any harm if they should self-ignite. After a couple of weeks the oil is hardened and produces no more heat. The rag is stiff and can be disposed in an ordinary waste bin. Another possibility is to store the rag spread out, for exaple over the oil bottle, so that the heat can get away. Self-ignition only appears with hardening oils in rags crammed to a ball where the heat can't get away from the middle.
    I sometimes store rags in an airtight container. In that way, there is no access for oxygen to the oil so it doesn't harden. In that way I can reuse the rag even after longer time.
   The reason for self-ignition: The hardening of the oil is a chemical oxidation process that produces heat. If a large rag is wet of oil, the hardening will produce significant amounts of heat. If the rag is crammed to a ball the heat can't get away from the centre because the rag itself is isolating and keeping the heat inside. Therefore, the temperature can rise to a level where self-ignition takes place. When the oil on a wood surface hardens it also produce heat, but the layer of oil is thin so little heat is produced and the heat can easily escape to the surroundings. Therefore, there is no risk of self-ignition on oil-treated wooden surfaces.


Sanding

Sanding of turned objects are usually performed with the obejct rotating in the lathe so that the lathe does part of the hard sanding job. Remember to remove the toolrest before sanding as it can lead to severe injuries. Also, remember to reduce the lathe speed significantly to avoid creating too much heat during sanding that can lead to scorching of the surface. Constantly move the sanding paper during sanding to avoid building up scratches from large sanding particles.

Sanding of the turned object is for some woodturners a sort of taboo: "If you are a good woodturner you can turn a surface that is so smooth that it (almost) doesn't need sanding". However, all turners use sanding and if you are less skilled, you may spend a long time sanding. Some skilled turnes state that you should only need sanding with a very fine grit if you are skilled at woodturning. However, I have heard of a professional turner that may start at grit 16 for his very large projects and it is common that professional turners start at grit 60! There are many opinions on sanding and I will write a little also.
    There are many types of 'sanding paper' and the prices are also varying a lot. Usually, the cheapest are useless for woodturning. Choose from the types made for wood. In general, types based on paper doesn't last as long (some are really bad) as types based on clothe backing, but these are also significantly more expensive. Some paper based types cracks significantly when bent (test in the shop). Some types are suited for wet sanding, but that is only relevant for wood if you want to turn and sand green (fresh) wood while it is still wet - usually wood is sanded when dry.
    It is usually best to use the sanding paper folded twice (in thirds) so that it has three layers. That gives the best stability and the best possibility of utilising the whole sheet (if the inner layer is moved to the front when it is going to be used). The three layers also isolate better to protect against the heat produced during sanding. But you should always avoid sanding so hard that too much heat is generated. It can lead to clogging of the sanding paper and scorching of the wood.

Use dust extraction! The dust created during sanding is unhealthy. You can use a vacuum cleaner if you don't have room or economy for a dust extraction system.

Don't use worn sanding paper! When the sanding paper is worn, it is as efficient as paper of a much finer grit but it may still make scratches in the wood according to the paper's nominal grit. So only use worn sanding paper if you really need to save money or you really love sanding.
    When the surface doesn't get smoother with the current grit, change to one with a 50-80 % finer grit. Before changing to a finer grit, you should remove dust from hollows and corners to avoid large particles from the first grits making scratches when sanding with a finer grit. This is particulary relevant with soft wood where the particles may get trapped in the wood.
    Sometimes there are areas that are difficult to get smooth, typically in end grain. Often, it is best to stop the lathe and sand these areas by hand.

It takes some experience to see when to change to a finer grit . Sometimes it helps to stop the lathe and sand a little by hand in a direction perpendicular to the sanding from the rotating wood. Then it is easier to see what is sanding marks from the previous grit and what is from the current sanding paper. Sometimes, it is necessary to step back one or two grits if it turns out, that the sanding with these hadn't been finished.

Sanding paper can also be used for shaping the wood. That is not reagarded as good practice, but if it is the easiest way to obtain the desired result, just do it. However, if too much wood is removed by sanding, the wood may become oval because the sanding paper removes more wood on side grain than on end grain.
    When sanding you should be careful to avoid rounding sharp corners or destoying the nice turned shapes by removing too much wood with the sanding paper.

Some objects can't be sanded using the power of the rotating lathe, for example square objects, objects with the bark on areas of the wood, deep narrow hollow forms etc. These must be sanded by hand or using power tools.

Power tools for sanding

Instead of holding the sanding paper by hand during sanding the rotating object, you may use a rotating sanding disc in a power drill. This can give a much faster sanding, so you have to be carefull or use a finer grit to avoid removing too much material. A rotating disc in a power drill is also very good for sanding objects that can't be sanded in the rotating lathe - often it is practical to sand the when still mounted in the lathe anyway.
    There are many types of sanding discs that can be mounted in a power drill. It is easy to obtain a good result with the types where the sanding paper is mounted on a foam pad. They are sold with foam of different softness, the softer are usually most useful, I think (for example 'Drechselstube'). The diameter is usually 5 or 7,5 cm where I find the smaller best as it is more difficult to utilise the centre part and the smaller can get into smaller corners. The foam can be cylindrical or conical. I find that the conical is best because it better gets into smaller areas.
    Usually, the sanding paper is mounted using Velcro so that it is easy to change between the different grits. The Velcro on the foam pad can be replaced when it is worn. The sanding paper sold for the sanding discs can have paper or clothe backing. I find the paper is fine because it isn't bent, but the glue holding the abrasive grains and the velcro backing has to be of good quality.
    You also can buy cylider shaped sanders for the power drill. A tube of sanding paper is held by air presseure (the cylinder can be pumped to pressure) or the cylinder can be made from a foam that is tensioned by a screw so that it presses against the sanding paper cylinder. The air cylinder is expensive but has the advantage that the hardness can be adjusted. The cylinder shaped sanders are less useful for woodturning.
    It can be an advantage to use a flexble bowden cable between power drill and sanding disc to give better flexibility and better acess. The bowden cable with sanding disc may also be mounted in a drill chuck in the lathe when sanding objets not mounted on the lathe.
    For sanding of the outside of a bowl, a sanding disc in an angle grinder at low speed is much better than a sanding disc in a power drill.
    Here are a couple of links to suppliers of sanding discs and cylinders: Linå, Sløjddetaljer, Craft Supplies, Drechselstube.

Wet sanding

Sometimes it is an advantage to sand a wooden object turned from green wood while it is still wet, for example if the object will warp much during drying. Then you have to use sanding paper for wet sanding and keep the wood really wet. Wet sanding can also be done with oil, but that is less useful and not at all for green wood. An advantage of wet sanding is that the sanding paper lasts much longer.


Pure oils:

Oils give a robust surface, but they may be washed away if they are washed regularly with soap as is the case with many kitchen utensils. This is especially the case for oils that do not harden. In these cases, it is necessary to refresh the oil with intervals.
    Oils fall in two distinct groups: hardening oils and non-hardening oils. Non-hardening oils have the disadvantage that the oil remains liquid and for a long time can make spots where the oiled item is placed. Furthermore, most non-hardening oils gets rancid making the wood smell after some time. Hardening oils react with oxygen to from a solid material after a shorter or longer period. After the hardening, there is no risk of spots where the item is placed and the oil doesn't get rancid. They are also much more robust to washing with soap.
    Oil is usually applied with a rag, sponge or a paintbrush. Paintbrushes can be cleaned in turpentine or kerosene. Rags and sponges can be kept in an airtight container for later use.
Linseed oil is made from flax seeds (botanical name: Linum). Read more (in danish) on the websites of Linolie Danmark ApS and Linolie 1-2-3. Linseed oil is my favourite finish for bowls and many other wooden objects. It's main disadvantage is that it hardens quite slowly - but it is still better than oils that doesn't harden. To speed up the hardening you may add siccative (drier) to the oil. Siccativeis sold in paint shops, but I don't use it much.
    There are different meanings about the toxidity of the linseed oil. In my opinion it must be safe to use for objects in contact with foods and for items that babies put in their mouth regarding that the oil itself comes from linseeds which you can eat in significant amounts and the oil hardens binding potential toxic contens. The toxic contents from linseed oil may origin from extraction of the oil using solvents so to be sure use virgin liinseed oil or cold-pressed linseed oil.
Boiled linseed oil is said to harden faster and penetrate the wood better but I haven't tried it. If it was so much better, it would be more widespread used, I think.
Tung oil (chinese wood oil) hardens faster than linseed oil and gives a slightly softer and more dull surface. It is said to penetrate wood better than linseed oil. It has a pleasant smell and I have used it a little, but it is more expensive and less available than linseed oil. It can among others be bought here: Sløjddetaljer, Craft Supplies.
Paraffin oil and 'Vaseline oil' don't I like but many do.
Various 'food oils': There are many types of vegetable oils used for food and most of these can be used for finishing of wood. I sometimes use rapeseed oil or sunflower seed oil. The food oils may have problems of becoming rancid making the wood smell. Some have tried many different of these oils and have found their favourites like walnut oil, thistle oil or peanut oil. Most of the food oils doesn't harden but a few are said to harden (I haven't tried), among others walnut oil and poppy seed oil.


Mixed oils

There are a lot of different mixed oils on the market and I here only describe the ones I use:
Bio-voksolie is made by the danish company Dorch&Danola. It contains wax and in many ways it has the advantages of both wax and oil. After a couple of coats it can be polished to a gloss like wax but it is much more robust. It dries fast, in about a day.
Danish Oil has some of the same qualities as 'bio-voksolie' but it dries faster and faster gives a more glossy (and a little greasy) surface. It also gives a slight darkening of the wood. Many companies produce a 'danish oil' and, of course, they vary, so the above isn't necessary valid for all. I use Rustin's Danish Oil for some decorative items.
Teak oil is the product name for furniture oils developed for teak (I don't include the products for garden furniture). They contain unspecified chemicals and solvents, so I don't use them for items that get into contact with food or babies. The products of different brands varies a lot. I think Sterling's teak oil is good for pine wood and I sometimes use it for homemade furniture.
Linseed oil with carnauba wax is a good alternative to commercial oils with wax. You can make it yourself by adding carnauba wax to linseed oil and heat it in a water bath until the wax is dissolved. Use about 20-30 g wax for 1 l oil, but the amount of wax is a matter of taste, depending on what type of surface you like. You may add siccative (drier) to get a faster drying oil, similar to bio-voksolie or Danish oil.
Linseed oil with beeswax is said to be good but I haven't tried it. A mixture of beeswax and carnauba wax in the oil is also possible.


Pure Waxes

Many types of wax can be polished to give a glossy surface but they are usually very delicate and sensitive to water. To give a more robust surface, the wax is usually applied after the wood has been saturated with oil and excess oil has been wiped off. The wax can be applied by pressing the solid piece of wax against the item rotating in the lathe. Pressing a rag against the rotating wood afterwards will spread and polish the wax. The wax may also be applied with a polishing disc where the wax piece is pressed shortly against the rotating polishing disc. The wood is then polished on the disc so that the wax is transferred to the wood. A third method is to soften the wax with a solvent and apply the soft wax with a rag. Afterwards, the solvent evaporates and the wax is polished.
Beeswax: I haven't used beeswax much. It gives a very delicate surface.
Carnauba wax is a hard wax produced from the layer of wax that is covering the leaves of the carnauba palm. It is very good as the last finish on a surface that needs a high gloss if lacquer isn't used. Carnaubavax can be bought here: Linå.
Wax mixtures of many sorts are sold. You can make your own. Beeswax and carnauba wax is said to be a good mixture that is easier applied than pure carnaubawax. If you add about 1/4 turpentine and maybe some linseed oil, you get a soft paste that can be applied with a rag. The commercial products are usually a mixture of different waxes and solvents to give a more or less hard paste that can be applied with a rag. I almost never use them.


Lacquer

Lacquer gives a surface that is robust to dirt. However, many modern water based products aren't robust to water. And all types have the problem that a crack or small hole will allow water to penetrate and destroy the surface. I don't use lacquer much because I don't like the feel it gives to the surface. But that is, of course, a matter of taste.
Shellac is secreted by the lac bug that lives in India and Thailand. The shellac is dissolved in spirits. I sometimes use shellac for small items like earrings. I simply dip the hole item in a thin shellac solution. Shellac is also useful when you need a fast drying lacquer, for example for leaf gold.
    Shellac solutions can be quite expensive but you can make it yourself. You can buy shellac flakes (from Sløjddetaljer or possibly from Aart de Vos) and dissolve it about 1:1 in spirits (may require heating). If the flakes aren't pure there may be a non-dissolvable residue and it may be required to filter the shellac solution.
Other types of lacquer of many types I use occasionally.


Preventing sun colouring

Many people want to keep the colour of the wood when it is freshly turned and before applying a protective finish. However, most finishes, if not all, influences the colour of the wood so that it darkens or gets more yellow. It isn't only the colour of the oil/wax/lacquer that changes the colour of the wood. Also the optical properties of the surface are changed so that the colour is influenced when the surface coat is applied. Water chages the colour of the wood too and water has absolutely no colour.
    The colour of the wood will also change with time becacuse of influence from sunlight and air. Light wood gets darker and dark wood gets lighter. Think of old oak forniture that is dark brown, almost black, but the newly cut oak is very light greyish. Or pine wood that becomes dark brown after hundred years.
    Some types of lacquer give a very small change in colour and some people chose these lacquers for that reason. If the colour has to be preserved, the lacquer must contain an UV filter to prevent sunlight from changing the colour of the wood.
    For some species of wood, a lye treatment can prevent darkening, but it also changes the colour of the wood so it isn't the way to preserve the original colour of the wood.


Bleaching

If you want the wood lighter than it is naturally, bleaching can be a solution. Chlorine is easy to try using a chlorine containing cleansing agent. It is more effective but also more unhealthy to use a mixture of 5-10 parts hydrogen peroxide and 1 part ammonia water that is applied to the wood. But be careful! Hydrogen peroxide can also be used alone. Another possibility is a 5-10 % solution of oxalic acid but it has to be neutralized afterwards.

Strong chemicals are avoided by using heavily diluted white paint or oil or lacquer with white pigment added sold for whitening wood are you can make it yourself.


Ammonia

Ammonia water can be used for colouring wood. It is quite well known that an ammonia treatment can give oak a beautiful brown colour, more or less the same colour that oak obtains naturally after hundreds of years. The method is simple: the wood is placed in a closed container and a small bowl with a little ammonia water is placed inside too. The container is closed and the ammonia vapours colours the wood. The longer time the wood is in the vapour the darker the colour. The container must be closed with a lid or a platic bag to keep in the very unpleasant vapours. Large items can be placed in a large plastic bag with the ammonia bowl inside.
    This method works very well with oak and also some other wood species but the result varies a lot. Some wood species like pine get a dull grey colour from the treatment looking much like natural ageing of the wood. Combined with sandblasting this can give an artificially aged wood.


Scorching

The flame from a torch can be used to colour wood. Depending on the amount of heat (time and distance), the colour can range from a sligt darker brown to black.
    Heavier scorching followed by brushing with a wire brush can be used to give an old look bringing the grain in the wood significantly out. Of course, this technique works best on wood with a coarse grain structure and large difference in hardness between the light and the dark part of the annual growth rings.


Sandblasting

Sandblasting can give patinated look to the wood, especially combined with ammonia treatment for woods that are turned grey by this treatment. Sandblasting equipment isn't standard and is is expensive and takes up space, so most people won't have it just for an occasional wood treatment. But you may have acess to the equipment somewhere.
    When sandblasting wood you have to be aware that blowing directly towards end grain gives a very rough craterlike surface. To avoid this simply blow from the side of the end grain.


Wire brushing

A wire brush can give a result that looks much like a sandblasted surface. Make a test on a scrap piece of the relevant wood species to see the result of different wire brushes.


Linseed oil/wood tar for outdoor use

For surface treatment of external wood I have been recommended to start with applying linseed oil in warm weather. When the oil has hardened, the wood is treated with wood tar diluted with 50 % linseed oil. This treatment should be done in hot weather to make the tar flow. This mixture is then reapplied to the wood every coulpe of years. I haven't tried it myself.
    More and more manufacturers are making wood tar. I have had a product from Borup Kemi recommended. Another possibility is from Morsø Maling.



You are welcome to send me an email if you have comments, corrections or additions.

Main page