How do I glue rubber so the repair does not just peel off?

Two things make rubber hard. It flexes, so any load on the bond arrives as peel, the mode in which every adhesive is weakest. And many rubbers contain plasticisers, waxes or release agents that migrate to the surface over time, so a rubber part that was clean when you cleaned it can be contaminated again by the following morning from the inside out.

Safety

What you'll need

• contact adhesive or rubber cement, or a flexible cyanoacrylate
• coarse abrasive paper, around 60 to 80 grit
• isopropyl alcohol
• nitrile gloves
• clamps, tape or a weight
• a cyanoacrylate activator for difficult rubbers

Step by step

1. Identify the rubber if you can.Natural rubber, nitrile and neoprene bond acceptably. EPDM — the black rubber of most roofing membranes, window gaskets and car door seals — is difficult and often needs a primer sold with the system. Silicone rubber effectively cannot be bonded with anything except more silicone.
2. Abrade the surface coarsely.60 to 80 grit, working the surface until it looks dull and slightly fuzzy. This is one of the few materials where a really coarse key helps, because you are creating mechanical interlock as well as removing the migrated surface layer.
3. Wipe with isopropyl alcohol and glue promptly.Plasticisers migrate back to a cleaned rubber surface within hours. There is no benefit to cleaning in advance, and there is real harm in leaving it overnight.
4. For a large flat area, use contact adhesive on both faces.Coat both surfaces, let them go touch-dry, then bring them together and press hard. Contact adhesive is designed for exactly this joint — a large flexible area with no clamping possible — and pressure, not time, is what makes it bond.
5. For a small close-fitting part, use cyanoacrylate.A rubber-toughened or flexible grade if you can get one. The join must be tight; superglue does nothing across a gap. Many rubbers accelerate cyanoacrylate dramatically, so you get seconds rather than a minute.
6. For a joint that must stretch, use a hybrid polymer.Hybrid MS polymer adhesives stay flexible after curing and are the only common household family that can be stretched with the joint. They are slower and gap-filling, which suits an imperfect fit.
7. Clamp or weight the repair for as long as you can.Rubber has memory and tries to return to its original shape, so it will pull itself off a fresh bond. Hold it flat or in position well past the handling time, ideally overnight.
8. Protect the edges of the repair.Peel starts at an edge. Feather the edge of the patch, tuck it under a lip if there is one, or run a bead of flexible sealant round the perimeter so there is no lifted corner for anything to catch on.

Tips

• A rubber patch cut with rounded corners lasts far longer than one with square corners, because a square corner is a stress raiser and a peel starting point.
• Vulcanising patch kits for tyres and inner tubes are a different chemistry again: the fluid softens and chemically fuses the rubber, so the patch becomes part of the tube. Where a kit exists for the job, it beats any general adhesive.
• Silicone rubber is bonded only with silicone sealant, and even then it is a seal rather than a strong joint. If the part is silicone, replace it.
• EPDM roofing has its own adhesive and primer system and the manufacturer's products are not interchangeable with general adhesives — using the wrong one is a common cause of a flat roof detail lifting.
• If a rubber seal keeps coming unstuck, ask whether it is being stretched to fit. A gasket cut short is being pulled by its own tension every minute of every day and nothing will hold it.

Common mistakes

• Using superglue on a large flexible area — It cures hard and brittle. The first time the rubber flexes, the rigid glue line cracks and the bond peels off from the crack.
• Cleaning the rubber the day before — Plasticiser migrates back to the surface. Clean and bond in one session or you are gluing to a fresh layer of oil.
• Stretching a rubber part to make it fit and then gluing it — The stored tension works on the bond continuously. Cut a new piece to length rather than stretching one that is short.

If it doesn't work

Questions people ask