Staying stuck: gluing tips

Above: Find ways to store clamps in orderly access.

Words and photos: Richard Vaughan

When a glue joints fails you can be sure it’s you and not the glue that caused it*. The range and strength of adhesives available today is remarkable but you can still get it wrong so here are some tips to help you not come unstuck.

Prepare your clamps

A little preparation saves a lot of hassle. When clamps are waxed they will shed drops of glue easily and the jaws will slide without catching. This treatment also prevents rust that can mar your work. Simply warm your clamps in the sun for half an hour or so, then bring them inside and rub paraffin wax into them while they are still warm. Once cool, rub them with a green kitchen scourer to push wax into the pores of the steel and remove the excess.

Waxing clamps and gluing on protective leather pads saves time.

Clamps that have bare steel jaws will certainly damage your job but instead of inserting pads each time it’s far easier to have leather pads permanently fixed to the faces of the jaws. File the steel flat and clean of paint, then use contact adhesive to attach leather with a couple of millimetres overhang all round. You can scavenge leather as offcuts from a friendly upholsterer, or leather belts and bags from Vinnies. A light coat of carnauba wax well rubbed in will resist accidental glue contact.

Use a clean and flat surface

Gluing up on a dodgy surface is inviting grief. If you can’t manage a dedicated table then you can easily make one when needed with sawhorses and hollow core doors (at about $30 or less each) to give a stiff flat surface, and then lay a sheet of melamine board over that. Glue squeeze-out is easily cleaned off melamine, especially if you wax the surface. I use carnauba wax because it sets hard so it won’t contaminate wood surfaces, meaning they won’t then resist finish or glue.

A roll of freezer go-between plastic gives a good barrier against glue getting onto clamps or benchtops as it resists adhesion to just about any glue, including polyurethane. Contact between clamp and a PVA glue joint will result in a deep black stain.

Easy-to-make stands like these are useful for a wide range of gluing applications.

Corner blocks improve the effectiveness of strap clamps when gluing mitred boxes. Made from ply, sealed and waxed to resist glue, they spread pressure evenly.

Glue tools

Shown above is the kit that I always bring out for gluing with PVA. Brushes from the $2 shop spread the glue and push it into the fibres and narrow spaces. On larger surfaces a piece of acrylic with a toothed edge works well, and if you are regularly laying veneer then a roller in a suitable paint tray is ideal. Storing the roller and tray in a plastic bag between uses saves on wash-up time as well as glue waste.

Don’t use steel tools to spread PVA because steel will react with the water in the glue and tannins in the wood to leave an obvious black stain line. However stainless steel palette knives are excellent for lifting the excess glue and smearing it onto the old phone book shown. Then the clean damp rag can be used to remove the final traces. Using the cloth to wipe without scraping first is likely to spread diluted glue around the joint, and this will show as an obvious discolouration when you apply the finish.

Note the nifty support for the brush that prevents it rolling away and getting clogged with sawdust mid job. The nails also serve as glue bottle cap rests because those caps have an irritating inclination to hide. Silicon egg poachers work well as glue bowls for when you don’t apply glue direct from the bottle, and they are so easy to clean

‘Which is the best/strongest glue?’ is a question I regularly hear and the answer is that it depends on the job.

Hide glue

Good old hide glue remains on my shelf because it continues to be useful, and not just for restoration work. Here are two examples of it being my first choice.

A commission to make a cabinet for display of military medals required baize being attached to the back (see above). Getting the right amount of PVA so the bond was strong but there was no bleed-through seemed a bit risky, and contact adhesive has the problem of no certainty that it will be perfectly flat and in position on the first attempt. So I coated the ply with hide glue, gave it an hour to harden, then used my workshop steam iron to reactivate the glue while flattening the baize into position. And should silverfish damage the cloth it will be a simple matter to iron and re-soften the glue then remove and replace the baize.

Hide glue spread on this base was allowed to harden, then heat from the steam iron enabled a smooth wrap around.

Another application was in making a display cabinet for a friend’s artwork. Hide glue spread on the base was allowed to harden, then the steam iron was used to stick the veneer to it. And as the curves on the corners were small radius the heat and steam softened the veneer to enable a smooth wrap around. Treating the veneer with softener then pressing it flat while it dried beforehand helped make the highly figured veneer more amenable. Note the wax pot I now use rather than the traditional double boiler.

Veneer softener makes highly figured veneer easier to glue. A beautician’s wax pot keeps hide glue liquid.

PVA glues

Polyvinyl acetate, better known as PVA, and generically as Aquadhere, very quickly supplanted hide glue from the 1950s and has been constantly improving and yet there are characteristics which many users are unaware of.

You must remember that PVA is not gap filling, and if you can fit a cigarette paper between the components then you have a gap that will weaken the joint. If you don’t have full contact then it is not the glue for the job.

Crosslinking PVA offers a glue which, unlike previous formulations is highly water-resistant. Although not technically classifiable as waterproof these are excellent for jobs where there may be exposure to moisture. They may be marketed as ‘exterior grade’ rather than specified as crosslinking.

Temperature and PVA

PVAs have a range of formulations and working times but time is generally very limited so it is not the right glue for glue-ups involving multiple joints. And in any case you must be fully prepared before you start spreading the glue. It is also temperature dependent — the warmer the workshop the less time you have.

Too cold for PVA is as much of an issue as too hot. You may be feeling hot as you busily glue up but if the wood and shed are way too cool you are set for failure. Some PVAs can handle temperatures under 10°C but I keep an eye on my maximum-minimum thermometer and won’t glue up with PVA if I reckon the temperature will go below 12°C during the hours the glue takes to fully set.

You need to be aware that there is a so called ‘green’ stage when the glue is still soft but has started to polymerize and if you move the parts to make adjustments at this stage you will weaken the joint and quite possibly have a failure.

The surfaces need to be wet when they are brought together. To ensure this I spread PVA on both surfaces and am very aware of the temperature because in hot weather the glue is likely to skin before you bring the components together. If this happens you can quickly brush another coat into the surfaces to refresh the glue. For open-grain timber you may well need to recoat the glue to allow for glue that has been absorbed into the wood. You should get a fine bead of squeeze out as you tighten the clamps to be certain there is enough glue in the joint.

Need more time?

For glue-ups that need a bit more working time, such as joining half a dozen boards for a tabletop, you will be looking to epoxy or polyurethane. Both have some gap filling capacity, less than a millimetre for the PU but epoxy can take up several millimetres (not that your work would ever need that of course).

For either glue you should be wearing disposable gloves as they do have nasty chemicals in them, and PU will stain skin black unless scrubbed off immediately. You just have to let it grow out.

Both these glue types should be applied with steel or plastic spatulas or spreaders rather than brushes which would need to be disposed of after one use. Both glues will set more slowly in cool temperatures but the chemical reaction that causes setting will not be otherwise affected.

Epoxy is nowhere near as straightforward to clean up as PVA so masking around the joint is a worthwhile timesaver. Remember to scrape off the squeeze-out and remove the tape before the glue hardens. If you should get epoxy on your skin you can safely remove it before it sets using household vinegar. I don’t recommend using vinegar to clean up around the joint however as it can cause many timbers to discolour.

PU does foam from the joint but is relatively easy to remove once it sets. Just the same, masking tape is still recommended.
Epoxy has a shelf life of several years when stored in moderate conditions but polyurethanes are far more limited as they are catalysed by moisture so once opened they start to expire and probably will be unusable inside a year.

The final word

For any glue-up you need to do a dry clamp-up, and it really doesn’t matter how small the job may be. Start by clearing the work area of anything not needed for the glue-up. Now you thoroughly check that joints come together snugly without force, that you have square, that you have enough clamps correctly set, and a method for locating and supporting components in more complicated assemblies.

And while you divert the phone to messages and have a quiet cup of tea you can consider what doesn’t look quite right as well as what you may have forgotten to do. Only when you are certain you are ready should you mask around joints to make clean-up easier, then disassemble and lay the clamps out in an orderly preparation for use. Phone off and gentle music on and you’re ready for a drama free glue-up.

Richard Vaughan is a furniture designer/maker in Brisbane who also teaches woodwork.