How do I glue metal to metal without welding?

Adhesive bonding of metal is a real engineering technique — modern aircraft and cars use it extensively — but it works there because the joints are designed for it, the surfaces are chemically treated, and the adhesive cures in an oven. At home you have none of those, so the honest ambition is a sound repair to something that is not carrying people or weight, made in a joint shape that plays to the adhesive's strengths.

Safety

What you'll need

• slow-cure two-part epoxy, or metal-filled epoxy putty
• abrasive paper or pad, 60 to 120 grit
• acetone and clean lint-free cloth
• nitrile gloves
• clamps or a vice
• a wire brush or rotary abrasive for heavy corrosion

Step by step

1. Check first whether adhesive is an acceptable answer at all.If failure of this joint could hurt someone, damage something expensive, or affect a vehicle's roadworthiness, it needs welding, brazing, riveting or bolting. Adhesive has no partial failure mode you can see coming.
2. Design the joint for shear, not peel.A lap joint loaded along its length spreads the load over the whole bonded area. A butt joint or a flexible strip being pulled back concentrates it on a line. If you can add an overlap, a sleeve or a doubler plate, do — it usually matters more than which adhesive you buy.
3. Maximise the bonded area.Adhesive strength is per unit area, so doubling the overlap roughly doubles the joint. A small area of expensive adhesive is weaker than a large area of ordinary epoxy.
4. Abrade both faces until they are bright and uniformly matt.Right through paint, plating, oxide, mill scale and corrosion. Cross-hatching the abrasion pattern gives a slightly better key than sanding in one direction.
5. Degrease with acetone, twice, using a clean cloth face each time.Metals arrive with cutting fluid, drawing compound or protective oil. Wipe, let it flash off, wipe again with a fresh cloth. If the second cloth comes away marked, keep going.
6. Bond immediately after preparing.Aluminium re-oxidises in minutes; steel begins to flash-rust in humid air within hours. The window between preparation and bonding is the part of this job people get wrong.
7. Mix the epoxy accurately and thoroughly.By the stated ratio, mixed for a full minute, scraping the sides and bottom. Slow-cure epoxies are considerably stronger and tougher than five-minute versions — use them wherever you can wait.
8. Apply thinly to both faces and clamp to a thin, even bond line.Squeeze-out all round is the sign of full coverage. Clamp firmly but not so hard that you press all the adhesive out. Leave it clamped for the full handling time and unloaded for the full cure.
9. Add a mechanical fixing wherever you possibly can.One rivet, screw or bolt through the overlap changes the character of the joint completely: it takes the peel loads, it holds alignment while the adhesive cures, and it gives a failure mode that is gradual rather than sudden.

Tips

• Metal-filled epoxy putty is genuinely useful for rebuilding a corroded section, filling a pinhole in a non-pressurised part, or making a stripped thread take a fastener again. It is not weld and it is not a pressure-vessel repair.
• Stainless steel and chrome plating are the hardest surfaces to bond because their passive layers are exactly what resists chemical attack. Abrade hard and bond immediately.
• Aluminium bonds well when freshly abraded and badly when not. It is the metal where the timing between preparation and bonding matters most.
• Heat destroys most epoxy bonds long before it destroys the metal. Anything near an engine, a flue, a hob or a light fitting needs a high-temperature adhesive or a mechanical fixing.
• If the parts do not meet, use a filled epoxy or a putty rather than more liquid epoxy — a thick liquid section is weaker as well as hotter as it cures.

Common mistakes

• Bonding to plated, painted or anodised metal — The bond is only as strong as the coating's grip on the metal beneath, and plating in particular releases in sheets. Abrade to bare metal.
• Using a five-minute epoxy for a structural repair — Fast epoxies trade strength and toughness for speed. Where the joint matters, the slow version is a genuinely different material.
• Loading the joint at handling time — Handling strength can be a small fraction of full strength. Metal joints in particular feel completely solid hours before they actually are.
• Repairing a car's structural, brake, steering or suspension component with adhesive — It is unsafe and it will fail an MOT. These are welded or replaced, and there is no householder version of that job.

Questions people ask