Finishing veneer and geometric intarsia

Why finishing veneer is fundamentally different from solid wood

With solid wood, a finishing mistake gives a second chance: the material is several centimetres thick, and hard sanding or scraping can expose a clean new layer. Veneer surfaces have no such reserve. A sheet of veneer 0.6 millimetres thick can only take a limited number of rounds of sanding before it gets too thin or the substrate shows through. So finishing mistakes on veneer have to be prevented by preparation, not corrected after applying.

A second difference is that the surface to be finished is not uniform. In geometric intarsia different species lie side by side, each with a different pore density, a different absorption and a different reaction to moisture. A uniform application meets a non-uniform surface. The result is different uptake, different drying and different reflection on each species, which changes the contrast, sometimes improving it, sometimes reducing it.

Third, applying the finish to veneer is an irreversible line in the making. What can still be sanded before the finish is sealed in by it afterwards. Glue residue, fibres not fully raised and scratches from the last round of sanding are made permanently visible by the first coat. No amount of finish solves this problem; preparation has to be complete before the first brush or cloth touches the material.

Preparing the surface and removing dust before the finish

The last grit before the finish sets the basic texture of the finished surface. For penetrating finishes such as oil and hard-wax oil, 180 to 240 grit is often considered a good range: fine enough for a smooth look, open enough for the finish to get in. If you sand finer, as Reihax does on Flow No. 2 up to 400 grit, you get a smoother surface that takes up a little less finish. Lacquered surfaces need finer preparation; 320 grit as the last pass makes sure no sanding scratches show under the lacquer. Details of the sanding sequence and the sanding tools are on the sanding page.

Removing dust is not optional. Sanding dust left on the veneer is embedded by the finish and stays visible as matt inclusions. The full routine: compressed air or a soft brush for cracks, joints and transitions at edges, then a lightly dampened linen cloth for the main surface, and finally a dry, lint-free cloth. Wenge dust needs particular attention: it is fine and pigmented and settles in the larger pores of neighbouring light species. At wenge-to-ash joints, the open pores of the ash have to be cleaned with compressed air after sanding, before particles are embedded by the moisture of the finish.

The dust in the room counts too. A freshly sanded workpiece standing open in a dusty workshop before the finish goes on collects visible dust within minutes. The finish should be applied in a clean, low-dust room, or the workpiece finished straight after the dust has been removed. Waiting and watching is no substitute for a clean room to work in.

Raised grain and damping beforehand

When water- or alcohol-based finishes meet sanded veneer, the wood fibres at the surface stand up. The finish swells them with moisture, and the freshly sanded surface that was smooth before applying becomes rough after the first coat. Once this first coat is dry, the surface is rougher than before. This effect is called raised grain, and it happens with oil-based finishes too, but is strongest with water-based lacquer.

The practical solution is damping beforehand. Dampen the veneer lightly with clean water after the last sanding, let it dry completely, at least twenty to thirty minutes, and then remove the raised fibres with 320 to 400 grit paper. This step raises the fibres once and sands them off while they stand up. The first real coat of finish then meets a surface whose fibres have already been raised and removed.

In geometric intarsia, damping beforehand is especially important, because the different species swell by different amounts. Wenge swells less than ash. If the ash raises strongly and the wenge hardly at all, a single final sanding creates a difference in level between the species that only shows after the finish. Damping beforehand evens out this effect before the decisive coat.

The first coat, uptake and sanding between coats

The first coat of finish shows how clean the preparation was. It brings out sanding scratches, glue residue and breaks in the fibre that were invisible when dry. If you find faults after the first coat, you are too late in the right order. The first coat is a revelation, not a rescue.

Uptake, how quickly and evenly the wood surface takes in the finish, varies greatly by species. Ash and oak soak up the first coat quickly and turn more matt at once, before the finish can be spread. With oil or hard-wax oil that means: apply quickly and evenly, do not go back over dry spots, do not spread over material that has already grabbed. A thin, even first coat that can soak in fully is better than a thick one that forms puddles and drying edges.

Once the first coat has dried completely, between six and twenty-four hours depending on the product and temperature, it is sanded between coats with 320 to 400 grit. The aim is to remove the fibres raised by the first coat, level dust inclusions and make an even base for the second coat. Sanding between coats must not be too aggressive: it should smooth the coat of finish, not remove it completely. On geometric intarsia with thin veneer, sanding off the whole first coat means the first coat has not built up the protection of the veneer. A touch test, in which a finger slides over the surface and should feel no more tack, shows whether the first coat has dried through.

Kinds of finish and how they work on veneer

Finishes work in two basic ways: penetrating or film-forming. Penetrating finishes, oil, hard-wax oil and wax, get into the pores of the wood and seal or occupy them from inside. The wood surface stays tangible, and the figure looks close and direct. Film-forming finishes, shellac, water-based lacquer and clear lacquer, build a glassy layer over the wood. They protect better, but the surface feels like a film, not like wood.

For geometric intarsia, where the quality of the wood and the contrast are the main elements of design, the decision usually leans towards penetrating finishes. A film-forming finish can visually cover the quality of the joints and give the object an even gloss that flattens the individual qualities of the different species. The depth that wenge or walnut shows after oiling is not the same as how these woods reflect under a film of lacquer.

The choice of finish also affects how well the finished object can be inspected. More on checking quality after the finish shows how the surface's behaviour in reflected light changes with the kind of finish.

Oil and hard-wax oil on veneer

Linseed-based hard oils and hard-wax oils are the most practical way into finishing veneer. They get into the pores, occupy them with cured oil and form no separate protective layer over the wood. The surface stays tangible like wood, and the figure becomes saturated and deeper. The contrast between dark and light woods usually rises after the first coat, because the dark wood gains a lot of depth while the light one mainly gets warmer.

Applying oil to veneer differs from applying it to solid wood: less material, less time to soak in. Solid wood can take several coats of oil and let it soak in over hours. On 0.6-millimetre veneer the amount of oil it can take is limited. Surplus oil that the pores cannot take settles on the surface and dries as a sticky, matt film. It can be wiped off with a clean cloth, but the timing is critical: wipe too early and you pull the oil out of the pores; too late and you leave a sticky film on the surface.

Hard-wax oil combines oil and wax. The oil penetrates, the wax seals the openings of the pores from above. The result is a surface that gives more protection against moisture than pure oil, but feels softer than shellac or lacquer. For desk objects and objects that are handled a lot, that is the preferred combination: protection from the wax, depth from the oil.

Shellac and wax for geometric intarsia

Shellac is a natural resin dissolved in alcohol that cures into a thin, hard, glassy layer. It builds depth in coats: each coat bonds with the one before through its alcohol, which briefly dissolves the layer underneath. After three or four coats the result is a depth that oil does not reach, especially on lively figured wood such as walnut or on the ray fleck of quarter-cut veneer.

Shellac on geometric intarsia has one specific advantage: it can be repaired in one spot. Alcohol dissolves existing layers of shellac; a new coat bonds with the edge of the old one if the work is clean. That is not possible with oil and lacquer. For objects where a local repair after years is realistic, shellac is the only finish that allows a seamless touch-up.

Pure wax is no protective system for geometric intarsia objects used every day. Wax on a raw wood surface penetrates very little and gives gloss and feel, but no protection against moisture. As a top coat over oil or hard-wax oil, wax gives a satin reflection that looks especially even in raking light. As a finish on its own for a desk object exposed to moisture and touch, wax alone is not enough.

Water-based lacquer and spray lacquer on veneer

Water-based clear lacquers have one advantage over oil: they do not yellow. Old oil finishes on light species such as ash develop a yellow cast over time, which reduces the contrast with wenge. Water-based lacquer stays visually neutral. The drawback is how it goes on: water raises fibres strongly, leads to more raised grain than oil and needs more rounds of sanding between coats to reach the final surface.

On geometric intarsia, water-based lacquer in several thin coats gives a surface that is acceptable to the touch with a careful sequence of sanding at 400 grit between coats, but visually carries the film character of a lacquer. Joints that look sharp and deep under oil look flatter under a film of lacquer, because the light is reflected at the lacquer surface and cannot get into the wood.

Spray lacquer from a can or a spray gun gives a more even thickness than brushing or wiping, but is hard to control on small geometric objects. Overspray, uncontrolled build-up at edges and the difficulty of starting seamlessly on a coat that has already begun to dry make spray lacquer more error-prone on small intarsia objects than on large areas. For prototypes or production series under controlled conditions it is an option; for one-off pieces with a craft standard, less so.

How each species takes up the finish

Wenge has large, open pores in a very dense wood. Oil turns it deep dark and brings out its pores and light stripes. But the dense surface between the pores takes up little oil; too much oil stays on wenge as a sticky layer that is slow to dry.

Oak has open, characteristic pores and a distinct pattern of rays. Oil gets in deeply, and the pores become visibly saturated. The result is a lively, structured surface in which the oil emphasises the pore structure. That is an advantage in design; but if you want an even surface, you will never get the same smoothness with oak as with a fine-pored wood. Grain fillers can close oak's pores, but they change its feel and look fundamentally.

Ash has large, open pores and takes oil readily. Its light colour mainly gets warmer and a little darker, and the pores stand out more. Because wenge gains a lot of depth in the same step, a wenge-and-ash combination usually has more contrast after the finish; next to walnut, on the other hand, the difference shrinks slightly. Planning has to take both into account.

Walnut lies between wenge and ash. It has medium-sized pores and takes up oil readily but not greedily. An oil finish on walnut gives a warm, silky surface that saturates the wood's warm brown without fundamentally changing it. Walnut tolerates more oil than wenge without leaving sticky residue.

Filling pores, open pores and building a film

A penetrating finish does not fill gaps. It occupies the pores with an impregnating layer but does not close them completely. The result is a surface on which the pore structure stays recognisable as a slight texture to the touch and the eye. That is no fault but the natural character of open-pored wood. If you want a completely smooth surface, you have to use a grain filler that fills the pores before the finish.

Grain fillers on geometric intarsia present a specific problem: they have to be neutral in colour or matched to every species involved. A grain filler that looks right on oak can look too light or too dark on the wenge next to it. Transparent fillers are the safer choice, because they do not affect the colour of the wood. The filling process, applying, working into the pores and removing the surplus, needs care at joints, because the filler can get into cracks at the joint and show as a white or light line after the finish.

Building a film with several coats of lacquer creates a protective layer that lies over the wood, not in it. Scratches in a film of lacquer go through the film, not into the wood. Repair means sanding off the film and building it again. On veneer that always risks the veneer thickness, because every round of sanding costs material. Once a lacquer film is built up, it should not be sanded off again as long as it works.

Darkening at the edges and treating edges

At the edges of a veneer assembly, where the veneer meets the substrate or another species, penetrating finishes behave differently from on the main surface. Edges are often like end grain: the veneer is cut across the fibre, and end grain soaks up more finish than long grain. The result is darkening at the edges that looks more intense than the neighbouring surface.

In geometric intarsia, darkening at the border between two species is an element that can be precisely controlled. At a wenge-to-ash border, the ash darkens more at its cut edge under oil than on its main surface. That widens the visible joint slightly: after the finish, the edge looks wider than the measured cut. If you work with very tight joints, you have to allow for this effect in the planning.

Exposed substrate edges, where MDF or multi-ply shows, cannot be treated with the same finish as the veneer. MDF edges soak up oil and lacquer without limit and never give a saturated surface. They have to be closed with edge veneer or paint before the finish. Open MDF edges under an oil finish look sticky, matt and unfinished after applying, a fault that no round of sanding fixes any more.

Glue residue and disturbances in the finish

Cured glue seals the pores. Oil and wax cannot get in where glue covers the surface. At glue stains still there after sanding, the finish creates a visible zone that shines differently from the surrounding wood: flatter, lighter, without depth. Invisible when dry; clearly visible after oil.

The most critical form of glue contamination, wenge pigment carried into light neighbouring fields, shows after the finish as a greyish brown sheen in the ash field next to the joint. Oil saturates the clean wood and makes the pigment stain stand out as a foreign zone that belongs neither to the wenge nor to the clean ash. Before the finish this stain is sometimes hard to see. Checking in raking light before the first coat is therefore obligatory; details are on the page checking quality in geometric intarsia.

Adhesive from veneer tape removed too late leaves a residual film on the show face that shows as a matt area after drying. This adhesive interacts with the finish like glue: oil and wax cannot get in. Because the adhesive is water-soluble, it is removed with a damp sponge before the finish goes on; after the finish, it can only be corrected by sanding off the finish and the contaminated zone of veneer.

How reversible finishing mistakes on veneer are

Layers of wax are the most easily reversed finish on veneer. Wax can be taken off with turpentine or white spirit, the character of the wood surface does not change, and a new coat of wax can be applied straight away. Layers of oil are harder to take back: the oil that has got in stays in the pores, but can be removed by sanding off the oil-impregnated layer of veneer. That costs veneer thickness.

Shellac can be dissolved with alcohol and so belongs to the correctable finishes, as long as the layer is still there. Water-based lacquer and clear lacquer can only be removed by mechanical means, sanding, once cured, which quickly reaches the limit of the material on thin veneer. A full lacquer film on 0.6-millimetre veneer is practically irreversible without endangering the veneer.

Yellowing from old oil finishes on light species is not fully reversible either, because the oil yellows in the pores. Sanding removes the yellowed surface, but the oil that got deeper stays. For objects where lasting neutrality of colour matters, especially in combinations of ash and wenge, water-based lacquer is the future-proof option, even if it is more demanding to apply.

How Reihax applies the finish

At Reihax the finish decides how an object looks and feels in the end. Geometric sharpness, the contrast between the woods and the feel of the surface are shaped by the finish, not just sealed. A finish applied too thickly bridges joints and visually softens the border between two species.

Every Reihax piece, the pen holders as well as the wall pieces, is finished with hard-wax oil in a satin sheen: two thin coats, with a light sanding between them with a fine abrasive pad. Reihax uses no lacquer and no shellac. How lacquer differs from hard-wax oil is described in lacquering.

Geometric sharpness is not created by the finish and must not be lost through it. A joint that was sharp after sanding has to be just as sharp after the finish. If the finish visibly widens a joint, through dark edges, different uptake in the pores or uneven build-up, the fault lies in the application. A sample with the same combination of woods shows this behaviour before the actual piece is put at risk.

The last check happens in raking light. Low light shows whether there are sanding scratches under the finish and whether joints reflect evenly. If the whole surface shines the same, with no difference between the species, too much film has been built up. The aim is a surface that throws back the light differently depending on species and fibre direction.

Offcuts from the same veneer make good test material. The contrast, darkening and edge effect of a finish show on them before the object is treated. That is especially worthwhile when wenge is involved, because wenge takes up finish to a different degree depending on the delivery.