Which superglue should I use — thin, medium or gel?

Cyanoacrylate cures when it meets moisture, including the trace of moisture on almost every surface and in the air. That single fact explains its speed, why it hates thick sections, why it grabs skin instantly and why it behaves so differently on different materials. Choosing between the grades is mostly a matter of matching viscosity to the gap you actually have.

Safety

Step by step

1. Look at the joint and judge the gap.Hairline where the parts genuinely touch, small where you can see a line, visible where you could slide paper in. That is the whole decision and it takes a second.
2. Use thin for a hairline.It is about the consistency of water and it travels along a crack by capillary action, so you touch the applicator to the edge of an assembled joint and it draws itself in. It cures in seconds and it has essentially no gap-filling ability.
3. Use medium for ordinary small repairs.Roughly the consistency of syrup. It stays where you put it long enough to assemble, tolerates an imperfect fit, and it is the sensible default bottle to own if you only own one.
4. Use gel for gaps, vertical work and porous materials.It does not run, does not soak into timber and fabric and disappear, and it bridges a visible gap. It is slower to cure, which is usually an advantage because it gives you time to position the parts.
5. Consider a rubber-toughened grade for anything that flexes.Standard cyanoacrylate is brittle and very weak in peel and impact. Rubber-toughened grades trade a little rigidity for a great deal of shock resistance and are worth having for anything that gets knocked or handled.
6. Use an activator only when you need it.Activator sprayed on one face cures the glue almost instantly and lets it set across a gap it could not otherwise fill. The trade-off is a more brittle bond and a slightly weaker one, so it is a tool for difficult joints rather than a routine step.
7. Use as little as will wet the joint.More glue is weaker glue with cyanoacrylate, because a thick section cures from the outside inwards and can stay soft in the middle indefinitely. Press the parts firmly together to squeeze the film thin.
8. Match the material as well as the gap.It is excellent on ceramic, rigid plastics such as ABS, hard rubber, metal and dense timber. It is poor on polypropylene, polyethylene and PTFE without a primer, it fogs clear acrylic and polycarbonate, and it stains porous stone.

Tips

• Keep the bottle in a cool, dry place — a sealed bag in the fridge genuinely extends its life, because the enemies are heat and humidity. Let it come back to room temperature before opening it or condensation will cure it in the nozzle.
• Bicarbonate of soda sprinkled onto wet cyanoacrylate cures it instantly into a hard mass that can be filed and sanded. It is a real technique for rebuilding a small missing corner, and it gets noticeably hot.
• White bloom around a joint is cyanoacrylate vapour settling on the surrounding surface. Clamping fast, using less, and increasing ventilation all reduce it; low-odour grades are formulated to avoid it.
• Wipe the nozzle and cap it immediately. Almost every bottle thrown away is one where the nozzle glued itself shut in the humidity of a kitchen drawer.
• The reaction with cotton and wool is genuinely energetic and has caused burns and started small fires. Never use it near either.

Common mistakes

• Using more to fill a gap — A thick pool skins over and stays liquid inside. It feels set, then fails weeks later when the load finds the soft centre.
• Using it on anything that flexes or takes impact — Standard grades are brittle. A rubber-toughened grade or a different adhesive family is needed for a part that bends or gets knocked.
• Using it on clear acrylic or polycarbonate — The vapour fogs the surface permanently and the bond is mediocre anyway. Solvent cement for acrylic; for polycarbonate, mechanical fixing or a specific adhesive.
• Storing an opened bottle in a warm humid place — Both heat and moisture shorten its life dramatically. A cool sealed container is worth several months of usable life.

Questions people ask