People choose adhesives the way they choose medicine: by reputation, in a hurry, at the point of need. The result is a cupboard of half-set tubes and a series of repairs that came apart.
Two ideas fix most of that. Joints almost always fail at the interface rather than in the glue, so cleaning and keying the surfaces matters more than which tube you bought. And a rigid adhesive on something that flexes will always fail, which is the single commonest mistake in household repair.
Preparation is most of it
Old glue must come off completely. New adhesive bonds to old adhesive, which is already failing, and the joint fails again in the same place. Scrape, sand or dissolve it.
Grease is invisible and fatal. Fingerprints, cooking oil, WD40, furniture polish and silicone all prevent bonding. Wipe with methylated spirit or isopropyl alcohol and let it flash off completely before gluing.
A thin coat is stronger than a thick one. The adhesive is the weakest material in the joint, so you want as little of it as will fill the gap. A thick glue line is a thick layer of the weak thing.
Cure time is not handling time. A tube saying "handles in 20 minutes, full strength in 24 hours" means the joint has a fraction of its strength at 20 minutes. Loading it early is how repairs fail.
Six that cover almost everything
PVA is the workshop default for wood, and it is genuinely stronger than the timber when the joint fits well. It needs a close fit and clamping; it fills gaps badly.
Epoxy is the one to reach for when the joint matters. Mix accurately, mix thoroughly for a full minute, and use the slow-setting version for anything structural because the five minute type is markedly weaker. It fills gaps, sticks to almost everything and is waterproof.
Cyanoacrylate, superglue, is fast and brittle. Perfect on a snapped rigid plastic part with faces that meet exactly. Wrong on anything that flexes. Adding a pinch of bicarbonate of soda to the joint sets it instantly and creates a hard filler, which is a genuinely useful trick for building up a missing corner.
Polyurethane cures by reacting with moisture in the air and the material, expands as it does, and sticks to nearly anything including damp timber. That expansion means it must be clamped or it foams the joint apart. Excellent outdoors.
Contact adhesive is the one people use wrongly. Coat both surfaces, wait until touch dry, then press together. It grabs instantly on contact with no repositioning, so alignment must be right first time. Use a sheet of paper between the faces and slide it out progressively if you need to place something large.
Flexible adhesives are the answer to most household breakages, because most household breakages are on things that move.
Matching glue to material
- Wood to wood: PVA, or polyurethane if damp or outdoors.
- Metal to metal: epoxy, or mechanical fixing, which is usually better.
- Ceramic: epoxy for structural, cyanoacrylate for a clean break in an ornament. Neither is food safe once you are eating off it, so a repaired plate is a display plate.
- Rigid plastic: depends entirely on the plastic. Polypropylene and polyethylene, which is most tubs, buckets and garden furniture, essentially do not glue and need welding or a mechanical fix. Test on a hidden area.
- Rubber, boots, flexible: flexible shoe adhesive or contact adhesive.
- Fabric: sew it. Glue on fabric is a temporary measure, always.
- Glass: epoxy, or a UV-cure adhesive if you have one. Never on anything that will hold hot liquid.
- Mixed materials: polyurethane or epoxy.
Clamping
The step people skip, and the difference between a repair and a failure.
- Cramps if you have them. G-cramps, sash cramps, spring clamps.
- A tourniquet: cord wrapped round the work and twisted tight with a stick, which is how chairs were glued for centuries and is still excellent.
- Tape, wrapped under tension, particularly for awkward shapes.
- Weight, for flat work: a stack of books, a bucket of water.
- Elastic, cut from an inner tube, wrapped repeatedly.
- Wedges driven between the work and a fixed point.
Protect the work with scraps of wood so the clamp does not bruise it.
Enough pressure to close the joint, not enough to squeeze all the glue out. A thin bead of squeeze-out along the line is the sign you have it about right. Wipe it off while it is wet, or on PVA, let it go rubbery and pare it off; wiping with a wet cloth pushes glue into the grain and shows up under any finish.
Safety
- Cyanoacrylate bonds skin instantly, and bonds eyelids faster than anything else. Do not work above your face. If skin is bonded, do not pull: soak in warm soapy water and roll the surfaces apart slowly. For eyes, do not attempt anything, cover and get medical help.
- Cyanoacrylate reacts exothermically with cotton and wool, which means it can get hot enough to burn and, in quantity, to smoke. Never use it to glue fabric, and never wipe a spill with a cotton rag.
- Epoxy is a sensitiser. Repeated skin contact can produce a permanent allergy. Gloves, every time, even for a small job.
- Polyurethane stains skin brown for a week and does not wash off. Gloves.
- Keep all of it away from children, and superglue in particular.
When glue is the wrong answer
Worth saying, because glue is the reflex.
- Anything structural that carries weight: a mechanical fixing, a screw, a bolt, a dowel or a metal plate is better, and glue on top of it is better still.
- Anything that flexes repeatedly: stitching, riveting or lashing beats adhesive.
- Anything in tension: glue is strong in shear and weak in peel. Design the repair so the load pushes the faces together rather than pulling them apart.
- Fabric: sew it. See hand-sewing.
- Polypropylene and polyethylene: mechanical fixing or plastic welding.
The strongest repairs are usually glue plus something else: a glued and screwed joint, a glued and stitched patch, a glued and pinned dowel.
Storing them so they still work
Half the tubes in a household drawer are already dead.
- Cyanoacrylate goes off once opened. Store it in the fridge in a sealed bag, cap clean, and it lasts months rather than weeks. Wipe the nozzle before capping.
- PVA freezes and is then useless. Do not store it in an unheated shed over winter.
- Epoxy has a long shelf life in two separate tubes and is the most reliable thing to stock.
- Polyurethane cures in the tube once air gets in. Squeeze the air out, cap tightly, store upside down.
- Check the dates, and write the date of opening on the tube in marker.