How do I prepare a surface so glue actually sticks?

Adhesion happens in the top few molecules of a surface. If anything is lying on those molecules — grease from hands, silicone from furniture polish, mould release left over from moulding, dust, a chalky bloom, an old adhesive film — then the glue bonds to that instead, and the joint is only as strong as whatever the contaminant is stuck to. This is why the same tube of adhesive can be brilliant one day and useless the next.

Safety

What you'll need

• isopropyl alcohol, methylated spirit or a proprietary degreaser
• abrasive paper, roughly 80 to 120 grit, or a sanding sponge
• clean lint-free cloth or paper towel
• nitrile gloves
• a stiff brush or vacuum for dust
• acetone, for plastics and metals that tolerate it

Step by step

1. Work out what is on the surface before you touch it.Hand grease and cooking film on anything in a kitchen. Furniture polish, which is often silicone and is the single worst contaminant there is. Mould release on new mouldings. Chalky bloom on old paint and old plastic. A clear film of old adhesive or sealant that you cannot see but can feel.
2. Degrease before you abrade, not after.Wipe with isopropyl alcohol or methylated spirit on a clean cloth, turning the cloth often so you lift the grease off rather than spreading it. Sanding first drives the contamination into the freshly opened surface where nothing will get it out.
3. Abrade to give the adhesive something to key into.80 to 120 grit on a firm block, worked in one direction. You are aiming for a uniform dull matt finish with no shiny patches left. Shiny patches are unabraded original surface and they are where the joint will start to fail.
4. Do not polish it.Finer is not better here. A surface sanded to 400 grit has less area for the adhesive to grip than one sanded to 120. This is the opposite of preparing for a finish, and people who are good at finishing get it wrong for exactly that reason.
5. Remove every trace of the abrasion dust.Brush, vacuum, then wipe again with fresh solvent on a clean cloth. Sanding dust is a release layer — the adhesive sticks to loose particles rather than to the substrate. This second degrease is the step people skip.
6. Let the solvent flash off completely.A minute or two is usually enough. Gluing onto a surface still wet with solvent traps it in the bond line, and it will keep the adhesive soft in the middle for a long time.
7. Test it with a drop of water.On a clean, well-prepared surface a drop spreads out into a flat film. On a contaminated one it stands up as a bead. Adhesive behaves the same way, so this is a free two-second check that predicts the result.
8. Glue promptly, and do not touch the prepared faces.A prepared surface starts picking up dust and airborne oil straight away, and a fingerprint puts back exactly the grease you just removed. Handle the parts by their edges and assemble within the hour.

Tips

• Silicone contamination is in a class of its own. If a surface has ever had furniture polish, a silicone-based cleaner or a smear of sealant on it, expect to have to abrade well past the original surface, and never use the same rag on two jobs.
• For plastics that solvent-weld, skip the abrasion and use the matching solvent cement instead — you are dissolving both faces rather than sticking them, and roughening does nothing useful.
• On metals, the freshly abraded surface starts oxidising within minutes. Abrade, degrease and bond in one continuous run rather than preparing everything first.
• Bare timber needs no degreasing unless it has been oiled or waxed, but it does need a flat, close-fitting joint far more than it needs sanding — a sanded surface that no longer mates is worse than a rough one that does.
• A cloth that has been used on the workshop bench is a contamination source. Use fresh paper towel per wipe on anything that matters.

Common mistakes

• Sanding first, then wiping — The grease is pushed into the newly opened pores where solvent on a cloth cannot reach it. The surface looks perfectly prepared and the joint still fails at the interface.
• Blowing dust off with an airline or by mouth — Compressor air carries oil and both carry moisture. A quick blast can add more contamination than it removes.
• Preparing a painted surface and gluing to the paint — The bond is then only as strong as the paint's grip on what is underneath, which on an old emulsion or a chalky exterior coat is close to nothing. Take the paint off where the adhesive goes.
• Assuming a new component is clean — New plastic and metal parts are frequently coated in mould release or drawing oil precisely because they are new. New is the most likely thing to be contaminated, not the least.

If it doesn't work

Questions people ask