In Feature Articles, Flexner On Finishing

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There’s no finishing problem more frustrating than glue splotches. You spend countless hours cutting, shaping, smoothing and joining pieces of wood only to have your work discolored where glue from squeeze-out or dirty hands seals the wood so your stain and finish can’t penetrate. The wood under the splotch doesn’t change color while all around it the wood is darkened.

Avoiding this common problem is easy with one of the following four steps.

• Keep the glue from getting on the surface of the wood in the first place.

• Wipe the glue off the surface while it’s still wet.

• Identify areas of dried glue and remove it before applying a stain or finish.

• Remove or disguise the glue splotch after it has occurred.

Preventing Glue Splotches

Glue squeeze-out is a good thing when gluing boards edge to edge, because it’s evidence you’ve applied enough glue and enough pressure with your clamps. This type of squeeze-out is seldom a problem, however, because you’ll remove all traces of it when you plane, scrape or sand the surface level.

It’s the squeeze-out from cross-grain joints, such as stiles and rails and legs and aprons, that causes problems because it’s hard to sand or scrape a 90° joint without leaving unsightly scratches.

The most obvious way to prevent cross-grain glue squeeze-out is to apply no more glue to the joint members (mortise and tenon, dowel and hole) than necessary to make a good glue bond. This is hard to do when working fast, however, because it’s difficult to avoid getting too much glue in the joint when you’re even more concerned about not getting enough to make a strong bond. The trick is to create spaces within the joint for excess glue to collect, giving you more leeway for how much glue you can apply.

To create these spaces, make your mortise or dowel hole a little deeper than necessary, chamfer the end of the tenon or dowel (most commercial dowels come this way), and chamfer the front edges of the mortise or dowel hole. When you then slide the joint together, excess glue will collect in the cavities before squeezing out.

To keep your hands clean of glue while gluing up, keep a damp cloth and a dry cloth nearby. If you do get some glue on your hands, wipe it off quickly with the damp cloth, then dry your hands with the dry cloth so you don’t cause grain raising.

Removing Wet Glue

The best time to remove excess glue is while it’s still wet by wiping with a cloth dampened with the solvent or thinner for the glue. To totally prevent glue splotching, however, you’ll have to soak the wood and wipe it dry several times so you thin the glue so much that not enough is left in the wood’s pores to cause a problem.

Some people remove glue squeeze-out by letting the glue dry just enough so it holds together and can be peeled off. This is a quick and easy way to remove most of the glue, but some will still remain in the pores and will have to be sanded, scraped or scrubbed out.

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