Beneath the Surface

Beneath the Surface

All too often, woodworkers have a fairly vague understanding of the various types of wood coatings and their particular methods of preparation and application. What should be clear to the maker is fitness of purpose for the furniture type, and based on this, a decision is made as to the type of finish and the gloss level required. It may even be that the piece, or pieces, are finished in more than one finish type. A dining table and matching chairs, for example, may all be oiled, with the exception of the table top, which the maker may choose to finish in a harder wearing, two-part spray finish. This article covers my particular method of oiling, and its preparation and finishing.

Why oil?
Over a long career designing and making, I have used most finishes, but have settled with an oil finish for almost all my work. The exceptions to this rule are cabinets and chests of drawers, where I apply a nitro-cellulose finish to all concealed parts to avoid the months-long off-gassing that oil finishes tend to do. The exposed surfaces are then oiled as usual. I mostly spray all internal parts of carcasses several days prior to assembly, masking off all joinery beforehand to allow the glue to do its job later upon assembly. Providing you choose an oil type that is formulated without any toxic drying agents, the task of applying an oil finish can be done without the need for cumbersome and sometimes costly gear. There are now a small number of manufacturers who produce oils that are mainly based on natural linseed oil, but which also contain only plant, or organic drying agents, rather than toxic chemical ones. We find that we are able to work with the natural finishes for extended periods of time without any discomfort, such as skin irritation, headaches or nausea. The Germans, in particular, produce excellent finishing oils for all woodwork, for both interior and exterior applications. Some are even specifically formulated as floor finishes for homes occupied by people with severe allergies. Natural finishes such as oil and shellac are best applied to hand-planed or finely sanded wood surfaces, unlike their modern lacquer counterparts that require a coarser surface to ‘key’ properly to. I prefer a surface that is so smooth that light is absorbed well below the surface, allowing all the wood’s natural, vibrant colour and figure to be reflected back to the viewer. Due to the rougher surfaces that are required on furniture that is sprayed, these beautiful, natural features in wood are mostly obscured. Lastly, oil finishes are so forgiving, both from the maker’s perspective and from the owner’s point of view, in that repairs to the finish, or even a complete rejuvenating oil finish, are dead easy years later.

Preparation
In a nutshell what you wish to achieve, ideally, with a wood surface prior to the first application of oil, is one that has all the attributes of a hand-planed surface. This is true irrespective of the nature of the surface, whether it be flat, or shaped. So, if your hand-skills are up to it, a finish straight off a well-honed cutting tool is, I believe, impossible to beat. If your preference is abrading, you would need to sand to about 600 grit, or finer. I generally sand through the grades 180/240/320 and 600, checking the surface regularly with raking light, which exposes scratches that show up as shadow lines. Imagine a scratch as a valley in an otherwise flat terrain, and the ‘sun’ (your light source) rising over the horizon.
The flat areas become bathed in light, while the valleys, or scratches, which may be nothing other than miniscule marks left by a coarser sandpaper, stand out as a shadow. This is the most efficient way to know when you’ve done enough, and neither too much nor too little sanding! The ‘feel’ test, I’m afraid, does nothing to inform you how you are progressing. I sand flat surfaces such as table tops with a random orbital sander, but always finish by hand, sanding with the grain in long, even strokes, before vacuuming out any dust residue from the grain. This last point is a very important one, as dust residue gives the wood an ill-defined, muddy appearance, rather than leaving it looking crisp and clean, with well-defined texture. I like edges, corners and fine detail to be sharp and crisply chamfered, so I always sand these areas entirely by hand to maintain definition, or use a sharp block plane or spokeshave. I feel that it is prudent at this stage to point out that I do not subscribe to the practice of wetting wood regularly to raise the grain, then sanding it flat. In fact, I feel this method, rather than achieving a nice smooth surface, actually creates one that is dimpled with tiny ‘divots’ that reflect less light, making the surface look and feel rougher. These little hollows in the surface are the spaces that were occupied by the tiny ‘splinters’ of raised grain caused by the drying out of the wood surface after it has been wetted with water. I find that with careful preparation of the surface of wood, it should never be necessary to de-nib the first, or any other, coat of oil. Of course, spray lacquers are different in that the first spray coat will raise the grain somewhat. I never use steel wool to prepare wood for a finish. Aside from the obvious problems caused by metal residue in sometimes tannin-rich woods, steel wool-rubbed wood loses crispness in its texture.

Applying the oil
Depending on the nature of the item and its surface, I use several methods to apply oil. For complex pieces like chairs I use a paintbrush, while a roller is used on big, flat table surfaces, and a rag is employed for small, simple items. Once a wet coat of oil is applied, the excess is always rubbed off, but the amount of oil that is applied, and the time between application and wiping off varies depending on a number of factors. The first coat
Once the wood is properly planed and/or sanded, and the dust removed from its pores, it is ready for its first coat. Polishing, needless to say, should always be done in a fairly clinical environment. Too much dust will invariably put paid to all your efforts, so a clean place with fresh, clean materials is a must. For the purpose of this article, I am oiling a jarrah (Eucalyptus marginata) dining table top. The four, subtly figured old-growth boards are consecutively quartersawn, with rich pink to burgundy tones. I opted to apply the oil with a simple paint roller, as this is the quickest way to apply the first, liberal wet coat. Remember that much of the first coat will be absorbed into the wood, so it needs to be the heaviest of all. Within several minutes you can see a patchwork of little dry spots where the oil has been absorbed into the wood. There is no harm in keeping it wet by going over the surface again with the roller for a little while, but don’t add any more finish. Once applied, I try to ‘read’ the situation as carefully as I can in terms of how much time I have before the oil begins to thicken. Ideally, I wait as long as possible before I rub this coat out, but this ‘window’ can be anywhere between 10 and 40 minutes, depending on the ambient temperature. Rubbing out a coat that has partially polymerised (the surface goes sticky) is hard work, and in such a case I would recommend re-wetting the surface with more oil to help dissolve it. The warmer the day, the quicker the oil thickens. A scrap piece of the same material, sanded to the same standard, is always a good option on which to test the surface, before rubbing any excess oil off. I simply rub a 50mm diameter ring on the surface with a rag-covered fingertip.
You want to feel a dryish oil covering that readily rubs down to the wood surface, leaving no residue. Too much pressure and rubbing means that it has been left too long! It is vitally important to remember that natural wood oils generate a lot of heat as they polymerise and can ignite, so place all rags and oil-contaminated materials in an air-sealed container to starve them of oxygen, if they need to be reused. Alternatively, place them in water and discard them after several days.

Rubbing
Any old, but clean, rag will do to rub off the excess first coat of oil. I gently rub across the grain to ‘fold’ some of the excess oil into the open pores to somewhat fill them, before rubbing along the grain in narrow figure of eights. I finish by rubbing, this time more vigorously, straight along the grain. There is a little secret to this last point: the rag gets folded into the hand a little like a large shellac rubber, and firmly held. Ideally, you want to rub in long continuous strokes from one end of the table top to the other, imagining your hand as an aircraft coming in to land early on the runway, and running well beyond the end, then ‘landing’ at the other end, and so on, overlapping each stroke a little. This way you avoid smudging the surface. Once the surface looks and feels smooth, it needs to be left for 12 hours or so to cure properly.

Burnishing
The first coat has now cured, and the next stage begins. Run your hand across the surface. It should feel as smooth as the raw, finely sanded wood. If it feels a little rough, it probably means that the oils, rags or atmosphere are contaminated with dust. In this case you are forced to sand this coat to de-nib it. If it is only very slightly rough, I rub it back with the back of a piece of #600 sandpaper, rather than the sandpaper itself. I now burnish the surface with a wad of natural hessian, rubbing hard and quite vigorously along the grain. You can wrap the hessian around a cork sanding block if you prefer. You should notice a distinct glossing of the surface. Be prepared for a workout, as this is pretty physical, but the results are astounding.

Second coat
Now that the first coat has sealed the wood and it has been burnished, all subsequent coats can be more sparingly applied, so an application rag and a wipe-off rag are all you need. Again, fold the rag into a neat rubber, charge it with oil, and wipe a wet coat across the grain, before wiping with the grain. This coat, and all coats that follow it, will go tacky much sooner, so be vigilant!
I tend to be cautious, so I will begin to wipe off all excess oil with the dry rag immediately, and then rub it with the hessian cloth to an even glossier shine. This time you’ll notice that the hessian gets dragged back by the curing oil, but keep it moving! The third coat and any further coats are all applied in exactly the same way as the second. If a satin finish is required after the last coat has dried, there are two ways I do it. 0000 steel wool can be used, or a special buffing pad fitted to a random orbital sander. The latter, especially, gives a beautiful, consistent finish. The steel wool method requires a little care to get right. Use only top quality steel wool, and tear off a decent wad of it, then wrap it around a cork sanding block and rub the surface only along the grain in continuous, straight sweeps. The aircraft landing technique should yield good results. A word of caution, however, is due: never use steel wool anywhere near open-grained woods such as wenge (Millettia laurentii) and especially oak (Quercus robur).

Oil recipes
I am often complimented on the finishes on my furniture, and asked whether I have any special recipe. Mostly, I use the oil just as it comes from the manufacturer, but if I’m after a heavier build, and/or a more water-resistant finish, I often add marine varnish and orange oil. The ratios will vary depending on the product used, but as a rule of thumb I mix propriety wood oil, marine varnish and natural orange oil in the ratio of 3:2:1, respectively. The varnish is a better sealer, while the orange oil helps to extend the mix’s open time, but also lubricates the rag when rubbing off. This system is particularly good for high-use table tops.

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