Making Amends

Regluing is fairly easy, but broken parts can be complicated.  A scarf repair is splicing new wood onto the old to make the broken part whole again. 

You probably spend most of your shop time building things from scratch, but now and then you may be asked to repair some existing furniture that is broken. Repairs usually involve regluing or fixing broken parts. 

Regluing is fairly easy for an experienced woodworker. In most cases it requires simply removing the old glue, applying fresh glue and clamping back together — the same as gluing up new furniture with the added step of removing the old glue.

Broken parts can be more complicated because they can’t always be glued back together successfully. Some of the part may be missing or the break may run across the grain. In these cases new wood has to be spliced on to make the part whole again. This type of repair is called a “scarf” repair. It’s the technique of patching or splicing a piece of wood onto the broken original at a long angle with the grain and then shaping it.

Broken tenons are usually best repaired by scarfing on new wood. In this case it was possible to do so without disassembling the rocking chair.
Bad repairs at joints can leave them so bunged up that the only way to repair them is to replace the entire part or scarf on new wood. Scarfing is less time consuming and makes color match easier because there is less wood to color.

A scarf can be used in a great many situations, the most common and beneficial of which occur at joints where tenons have broken off or where mortises or dowel holes have been bunged up by previous repairs. The scarf replaces all the missing or damaged wood with sound wood so the joints can then be recut.

There are, of course, other ways to replace missing wood. In some cases, as with edges of table tops or corners of case furniture where the wood isn’t structural, you can use an epoxy- or polyester-patching product to build out the damaged areas.  But with joints, adding wood is the only repair that’s strong enough to hold up in use.

You could also replace the entire part, and this is often the best way to go. But there are four situations when it’s not:

•  Significantly less work is required to scarf on a piece of wood  than to make an entirely new part.

•  Adding a patch can be done without having to disassemble the furniture.

•  Making a new part will create color-matching problems while scarfing will avoid them.

•  The piece has historical value, and you’re able to retain more of that value by scarfing than by replacing the entire part.

To make a scarf repair first cut away the damage at a long angle to the grain using a bandsaw. 
Hand-plane the rough scarf cut perfectly flat. If the part is still attached to the furniture, it can sometimes be a challenge figuring out how to hold the furniture steady. 

Scarfing theory

Scarfing works because wood bonds well face-grain to face-grain but not well end-grain to end-grain or end-grain to face-grain.  (This is the reason you can’t just butt boards together to make furniture; you have to cut mortise-and-tenons, dowels or dovetails.) Therefore, when performing a scarf repair, you need to cut the parts at the longest angle possible with the grain.

Once you have determined the angle you want to cut, draw a line and then make the cut on a bandsaw.  This will leave a rough surface, but it’s easy to clean up and flatten using a hand plane.

Possessing the skills to tune, sharpen and use a hand plane are essential to accomplishing a scarf repair. It’s rare that a machine tool can be used to create a perfectly smooth and flat surface on an already existing part.

With the piece planed flat, cut a negative (a piece corresponding to the wood you have removed) from wood of the same species and with a similar grain pattern.  Use old wood if possible when working on old furniture to make color matching easier and to reduce the stresses from uneven shrinkage and swelling.  Old wood has “settled down” more than new wood.

In almost all cases, you should make the negative thicker and longer than it needs to be and then trim and shape it after it is glued up.  Don’t try to make an exact replacement, because it will probably slide a little during the glue up.  A longer piece of wood is also easier to hand-plane perfectly flat than a very short piece.  In fact, if the piece is long enough, you can use a stationary jointer to flatten it.

The trick to making the negative is to line up the grain accurately.  An easy method of doing this is to hold the original part with the angled cut already made up against the board from which you’re going to cut the patch.  Line up the grain of the two pieces and then draw a line for the angle you’re going to cut.  Extend the line at both ends so the patch will be longer than necessary.

With the long angled cut planed perfectly flat, lay out the angle on the replacement wood by holding the original up against it and drawing a line. 
Glue the replacement patch to the original. Framemaker’s spring-miter clamps have built-in teeth for a firm grip and are handy for reducing sliding during glue up.  
When patching an existing part , it’s very helpful to draw an outline of the part using the original pieces. You can then use this outline to cut the exact shape.
After drawing an outline of the part, plane the surface perfectly flat and glue on a patch.

Gluing up

Before gluing up, check that you have the angled cuts on both the original part and the patch planed perfectly flat.  Hold the two up against each other with a light source behind them. If you can see any gaps, the parts aren’t flat.  Remember:  You need tight wood-to-wood contact the entire length of the joint to achieve the best bond.

When you’re satisfied, glue the parts together.  To keep them from sliding, insert a brad or use framemaker’s spring-miter clamps gripped at a 90° angle to the plane of the joint.  These clamps have teeth or points to hold the parts in place so they don’t slide. 

After the glue has dried thoroughly, remove the clamps and shape the patch using whatever tools are most efficient for the purpose. I find a hand, or “card,” scraper very useful for the final shaping of most scarf repairs.

— Bob Flexner is author of “Understanding Wood Finishing,” now in its second, fully revised edition.

Use the outline  you have drawn to help shape the part, then rebuild the joint using whatever tools necessary. In this case dowel points are used to identify where the dowel holes are to be drilled.
This broken chair rung  had been previously glued, but there was too much end grain for the glue to hold. Scarfing instead of turning an entirely new rung saved more of the original color making the final color matching easier.

Here are some scarfed spindles from the back of a chair in various stages of repair.

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