Blotchy glue marks after finishing

How to prevent glue stains from ruining finishes. October 2, 2001

We recently built an oak bookcase. After finish had been applied, the glue showed up. The glue had been wiped off and a tack rag used on the piece before finishing. What can we do?

Forum Responses
A good furniture touch-up artist or spot-finisher should be able to color and blend those blemishes for you.

When you clamp two pieces of wood together, glue will squeeze out. Do not wipe this glue off. Let it form a skin on the little bulges (1-3 hrs) then take a chisel and carefully go down the joint at a right angle to that joint with the sharp edge of the chisel in kind of a stabbing motion, in and out. Now, pull the rubbery strip off with your fingers and still resist the temptation to wipe any off. You will need to do some minor sanding when it's totally dry.

When you wipe glue on the surface of wood, you push it into the grain, where it dries invisibly. Then, when you stain the surface, the stain will not penetrate the grain that's sealed with glue. The result is ugly blotches.

There is no fast and easy way. The glue must bulge out to get a completely glued joint and no glue can be pushed into the grain.

To sand correctly, you must do so uniformly and the only way to see how well you've done it is to reflect light off of the surface from more than one direction. A single direction will almost never reveal a poorly sanded portion. This is because the lines you see that need sanding are really shadows formed by the lights that are shining on it at that particular angle. And by definition, a swirl that needs sanding has some part of its direction that won't cast a shadow from a single light source.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment form contributor A:
I have a method to check for glue, even along the joints, etc. prior to finishing. Shine a black light on the wood. The glue will glow a yellow/green color. If you have glue on the wood you will at least know about it before you apply the final finish. To help prevent glue getting onto the finished surface, first of all apply the glue sparingly and avoid getting close to the edges. I use a warm water soaked cloth to wipe off any excess that squeezes out during assembly. Then leave it alone and cut or chisel works real good if it only squeeses out along a join and does not touch the adjoining finished surfaces. Be real careful. Save some materials and use glue sparingly.

Comment from contributor B:
There is only one way to prevent ugly glue stains from appearing once wood is finished, and that is to ensure you have cleaned off all glue whilst it's wet. Immediately after clamping rub off excess glue with a damp cloth, then scrub that area with hot water and a toothbrush is excellent at cleaning out the grain. Dry off area with dry cloth and when fully dry sand area with a firm to hard block, perhaps wedge-shaped in order to gain best access to corners.