Workshop

Fixing mistakes

Mistakes happen, even with careful work. Whether they can be fixed depends above all on when they are found. This page shows how open joints, blisters, tear-out, sanded-through spots and glue stains are repaired, and how to tell that starting again is the better repair.

Diagram of four moments at which a fault is found: dry fit, after pressing, after sanding and after the finish, each with the repairs still possible and rising effort — The later a fault is noticed, the fewer repairs are left and the more they cost. Before pressing almost everything can be undone.

First classify, then repair

Before repairing, ask two questions. The first: what stage is the piece at? Before pressing every piece still lies loose, and almost every fault can be put right by cutting again or swapping. After pressing every piece is fixed, and every repair costs veneer thickness, time or both. After the finish there is also the surface, which has to be renewed with every repair.

The second question: where does the fault come from? A fault that sits in a single piece can be fixed at that piece. A fault that comes from the design, the template or the substrate, on the other hand, is in the whole piece and often in every piece of a batch (working with templates). Repairing it at the surface only treats the symptom. How to stop faults arising at all is described in avoiding mistakes in intarsia; how to find them early, in checking the quality of an intarsia.

Open joints

An open joint is a gap between two pieces through which you can see the glue or the substrate. With high contrast, between wenge and ash for instance, even a gap hardly wider than a hair shows.

Before pressing: the piece is cut again or replaced with one that fits better. That is the only repair that leaves no trace.
After pressing, narrow gap: a thin sliver of the same veneer, ideally from the same sheet, is pressed into the joint with a little glue, cut flush once the glue has set and levelled with the next sanding. Its grain runs like that of the piece it belongs to.
After pressing, wide gap: one of the two pieces is cut out and replaced, as with a sanded-through spot.

A filler of sanding dust and glue is a common emergency solution, but rarely a good one. The glue binds the dust more densely than the wood grew, and under the finish the joint almost always turns darker than its surroundings. In a light area that is a visible line, in a dark one sometimes acceptable.

Joints between light and dark veneers of Flow No. 2, pressed and not yet sanded — Pressed joints before sanding. This is where you see whether a joint is closed; sanding does not make a gap smaller.

Blisters and lifted spots

Where there was too little glue or too little pressure in pressing, the veneer does not hold to the substrate. Such voids show as a slight bulge in raking light, as a spot that gives in sanding, and as a hollow, brighter sound when tapped with a fingernail. They should be found before the finish, because a finish on a blister only seals in the fault.

Diagram in three steps: slit the blister along the grain with a scalpel, get glue under it with a syringe, press it flat with a caul and release film — Repairing a void: slit it along the grain, get a little glue in, press it with a caul and release film until the glue has set.

Open it: slit the blister with a scalpel along the grain. A cut with the grain later disappears into the figure of the wood; a cut across it stays visible.
Add glue: with a syringe or a thin blade, get a little glue under the veneer and spread it by pressing lightly. Wipe off whatever squeezes out at once.
Press: release film or baking paper on the spot, a flat caul over it, then pressure with clamps or a weight until the glue has set. Only then is it sanded.

A lifted edge, at the edge of a surface or the corner of a carcass for instance, is treated the same way: get glue under the edge with a thin blade and press. With some glues a void can also be laid down again with heat, with an iron through a piece of paper for example. Whether that works depends on the glue, and heat can release neighbouring areas too. Which glue reacts how is covered in glues for veneer and intarsia.

Tear-out and damaged edges

Tear-out happens in cutting, when the blade pulls fibres out instead of cutting them, or in sanding, when a sharp piece catches at the edge. Brittle, coarse-fibred woods such as wenge are especially prone to it.

The wenge area of Flow No. 2 before sanding with visible tear-out and scratches — A real fault from the Reihax workshop: tear-out and scratches in the wenge area of Flow No. 2, before sanding. Only sanding shows how deep they go.

The splinter is still there: it is put back with a drop of glue and pressed briefly. That is the best repair, because colour and grain match by themselves.
Shallow tear-out: if it is no deeper than the allowance for sanding, it disappears in sanding. Whether that is so only sanding shows; but the veneer thickness must not be sacrificed for it.
Deep tear-out: a small patch of the same wood is set in, aligned with the grain. In a pattern it is better to replace the whole piece than to set a patch in the middle of an area.
Damaged edge on the carcass: a knocked veneer edge is reglued if the veneer is still there, or replaced with a narrow strip of the same wood.

Sanded-through spots

Veneer is thin, and at edges, corners and spots that lay a little higher in pressing it is quickly sanded through. The substrate or the glue line appears underneath. Further sanding only makes it worse, and paint or stain rarely hide it convincingly.

The clean repair in intarsia is to replace a whole field. The pattern gives the cutting lines: the affected piece is cut out along its joints with a sharp knife, the old glue carefully removed from the substrate and a new piece of the same wood fitted. Because the surface around the piece has already been sanded, the new piece stands proud at first and is carefully brought down to its level. That is fine work, but the result is a field like any other. How to avoid it is described in sanding veneer.

Glue stains and dust in the pores

Glue pushed up through joints or pores in pressing is hard to see on raw wood. Under oil or lacquer it stays as a light, matt spot, because the finish does not soak in there. You can see it beforehand by wiping the surface with a damp cloth or a little spirit: where there is glue, the wood does not get darker.

Glue on the surface: scrape it off with a card scraper or a sharp chisel and then sand. Sanding alone often just smears the glue further.
Dark dust in light pores: sanding dust from wenge or walnut rubbed into ash or oak sits deep. It can only be removed by taking material off, and only as far as the veneer allows. That is why the dust is removed between the stages of sanding.

Faults that only the finish shows

The finish strengthens everything the surface had before: scratches, sanding marks, glue stains and differences in height at joints. That is why the check after the last sanding matters most. What still shows after that is treated according to the finish: an oiled surface can be sanded locally and re-oiled, while a surface built up with lacquer or shellac usually has to be rebuilt over the whole face. Which surface can be touched up how is covered in repairing a finish, finishing veneer and oil, wax and shellac.

Flow No. 2 after sanding with 400 grit, before the finish — Flow No. 2 after the last sanding: the right moment to check joints, edges and areas in raking light, before the finish fixes every fault.

When starting again is better

A repair is worth it when it is smaller than the fault and less visible. Starting again is the better choice when:

the fault comes from the design, the template or the file and is therefore everywhere,
the substrate is warped or comes away from the veneer over large areas,
several faults come together on one piece, each of which could be repaired on its own,
the repair itself would stay visible as a spot, a line or a difference in colour.

In a small batch this decision is often easier, because material, file and preparation for another piece are already there. And a rejected piece is not lost: it shows at which step the fault arose, and so where to check more closely next time.

Checklist before any repair

Stage decided: before pressing, after pressing, after sanding or after the finish
Cause found: a single piece, or the design, template or substrate
Repair material of the same wood, ideally from the same sheet, grain aligned
Remaining veneer thickness checked before sanding
Glue used sparingly, squeeze-out removed at once, pressed with a caul and release film
Surface dampened after the repair and checked in raking light
Decided honestly: is the repair less visible than the fault?

Frequently asked questions

How do you repair an open joint in intarsia?

Before pressing, the piece is cut again or swapped. After pressing, a narrow gap can be closed with a thin sliver of the same veneer, set in with glue and sanded flush. A filler of sanding dust and glue is only an emergency solution, because it almost always turns darker than the wood.

What do you do if veneer comes away from the substrate?

A blister is opened with a fine cut along the grain, a little glue is got underneath with a syringe or a thin blade, and the spot is pressed with a flat caul and release film until the glue has set. A lifted edge is reglued the same way.

Can sanded-through veneer be repaired?

Not invisibly by sanding further, because the substrate lies underneath. The usual repair is to cut out the affected field along its joints and set in a new piece of the same wood. In intarsia it helps that the joints of the pattern already give the cutting lines.

When is starting again better than a repair?

When the fault lies in the design, the template or the substrate, when it affects large areas, or when the repair itself would stay visible. A piece made of many repairs is rarely as calm and precise as one built anew.