If you’ve never had the experience of applying the first coat of finish to a project and having some of the finish fail to “take” because glue has sealed the wood grain underneath, go read something else. What follows is intended for the vast majority of woodworkers who have encountered this problem.
Most often this glue is leftover “squeezeout” (glue squeezed out of joints when they were clamped during assembly). Unless you’re Norm Abrams (who never has squeezeout, but he never has dust in his shop either; doubtless the result of some Faustian bargain), squeezeout is an inevitable consequence of joinery. Unless it’s dealt with properly, it’s almost guaranteed to cause problems during finishing. If a squeezeout problem is first detected at the finishing stage, correcting it is complicated by the fact that fully cured wood glue is harder than the surrounding wood, making sanding, scraping, chiseling, or planing difficult, particularly on inside corners. Sanding away fully cured glue may also result in a high spot where the glue was, and even after the offending glue is removed the finish may not take uniformly.
It’s far easier to deal with squeezeout while the project is in its assembly stage, well before the glue has fully cured. The following hints assume that you’re using a water soluble aliphatic resin emulsion (aka “yellow glue”, such as Titebond II). The materials you’ll need are a supply of glue sticks, clean water, paper towels, scrapers, and a light weight cranked-neck chisel (the bent handle allows the back of the blade to be pressed flush against the wood). The best scraper is a cabinet scraper (more on this below), but it may be necessary to use something smaller in tight spaces. My personal favorite for cramped quarters is a blade measuring about x 2 inches taken from a miniature block plane. Whatever your choices of scraper, make sure they are properly sharpened.
1) Remove the excess sawdust from the material to be glued, but don’t remove it all, and don’t use a tack cloth. Yellow glue has high surface tension. A little dust on the surface won’t affect the strength of the joint, but it will reduce the amount of squeezeout that comes in direct contact with the wood.
2) Apply enough glue to thoroughly wet the joining surfaces, but don’t overdo it. On the other hand, don’t skimp either. The objective is a strong joint, and to achieve this some amount of squeezeout is unavoidable. The difference between dealing with a small amount and a moderate amount of squeezeout isn’t worth worrying about.
3) Assemble the work and clamp as necessary. It’s usually possible to remove the clamps after 45 minutes, which allows adequate time to deal with most squeezeout. But if it’s necessary to leave the work clamped much longer, you’ll need to find a way to work around the clamps. Don’t leave the work clamped overnight and expect to deal successfully with squeezeout the next morning – the glue will be too hard by this point.
4) Inside corners: Since these can’t be effectively sanded once they’re assembled, inside corners should be dealt with as soon as the work is clamped. Use a glue stick to remove the worst of the squeezeout. Keep the stick clean by wiping it frequently on a piece of scrapwood, then on a moist paper towel. When the stick comes up clean, wet the squeezeout area with a clean moist towel. Get the area damp, but don’t soak it. Then use a scraper, getting it all the way into the corner, using firm pressure, and pulling it in the direction of the grain to remove everything that’s left. Wipe the scraper on a moist towel after each swipe. What will come off initially is a mixture of glue, sawdust, moisture, and some fine shavings. After a few swipes only fine shavings will be taken off, and the heat generated by the scraping will have evaporated most of the moisture. You’ll quickly get a feel for when you’ve scraped enough – usually half a dozen swipes is sufficient. Deal with one joint at a time, and work reasonably quickly. It usually takes me just a minute or two to do an inside corner. Due to the glue’s high surface tension, the clock doesn’t really start until you’ve first disturbed the squeezeout, so it doesn’t matter if a joint has sat for a few minutes before you start to work on it. When the work is unclamped, rescrape the area in order to flatten any grain raised by the moisture, get a good light on it, and brush on some paint thinner. If there is any remaining glue (nine times in ten there won’t be), it will show up white. Scrape as necessary until it passes the paint thinner test. Since scraped surfaces may take finish (especially stains) differently than sanded surfaces, hand sand the scraped area using a sanding block with sharp corners and the same grit as the rest of the work. [A cabinet scraper is the ideal tool for removing squeezeout from inside corners. I put the hooks on the short sides of a thin scraper. Short sides so I can apply more pressure, and thin because it gives me a better sense of the underlying wood being scraped. Glue quickly fouls a cabinet scraper, so hooks have to be reformed frequently. You’ll know when it’s time – you’ll get dust, but no fine shavings.]
5) Outside corners on dovetail (or finger) joints: End grain can wick up glue and seal the grain for some distance below the surface, so more material may eventually have to be sanded off the pin piece (the piece with exposed end grain) than the tail piece (which shows only flat grain). If the two pieces start out flush when they’re assembled, the result after final sanding may be a tapered corner. So I make a practice of cutting and assembling dovetails so that the pins (the end grain) stand a couple hundredths of an inch proud of the tails. After the clamps are removed use a cranked-neck chisel held flush to the pins to remove the squeezeout from the exposed end grain. This will also remove most, but not all, the glue from the tails. Don’t worry about what’s left – final sanding will take care of the remaining glue and will automatically sand off more end grain than flat grain. The corner should therefore still be square after all the glue is gone and the joint passes the paint thinner test.
6) Everything else: After the clamps are taken off, use a cranked-neck chisel to get under squeezeout – it should come off as a partially cured ribbon of glue. Use a scraper if necessary, but don’t worry about passing the paint thinner test at this point – any small amount of squeezeout that remains will come off easily during final planing and/or sanding. Make sure the joint passes the paint thinner test when you’re done.
Like most woodworking techniques, all of the above quickly becomes automatic. You’ll soon be able to tell when the glue is gone just by the feel of it, and flunking the paint thinner test will become a rarity. Since I started using this method, I’ve never had a glue problem survive to the final finishing stage. I can’t claim this is the best way to deal with squeezeout, but it’s the best way I know, and it works. Good luck.