Metal bonding fails at the oxide and the oil, not at the logo on the tube. A “steel-reinforced” epoxy on mill scale with fingerprint oil is a weaker joint than a cheap epoxy on bright, scuffed, degreased steel. Treat the numbers below as a chemistry filter from our matrix, then spend your effort on prep.
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Matrix snapshot (metal substrates)
| Adhesive type | Steel | Aluminum | Notes |
|---|---|---|---|
| Two-part epoxy | Good (3000 psi) | Excellent (6000) | Default structural choice; fills gaps. |
| Steel-reinforced epoxy | Good (800) | Excellent (5000) | Built for metal repair; machineable when cured. |
| Metal epoxy | Good (750) | Excellent (4500) | Same family; check temperature rating on the pack. |
| Cyanoacrylate | Good (700) | Excellent (7000) | Close fit only; brittle in peel and impact. |
| Epoxy resin | Good (700) | Good (5000) | Use a structural (filled) grade, not a craft coating resin. |
| Silicone | Poor (150) | Fair (1000) | Sealant, not a machine mount. |
| PVA / Titebond | Poor | Poor | Wrong material class. See Titebond for wood. |
Aluminum scores look heroic because a good epoxy wets alumina. Stainless and galvanized need the same prep discipline as mild steel, plus an extra degrease — zinc soaps and drawing oils hide on new parts.
Pick the chemistry from the load
- Shear, gap, outdoor, “I wish I could weld this”: two-part epoxy. J-B Weld is the household name in that slot.
- Tiny, flush, indoor, no peel: a gel cyanoacrylate. Think of it as a clamp you cannot see.
- Dissimilar metals that will galvanically corrode: epoxy still works as a barrier if the joint stays sealed; do not use CA as a corrosion strategy.
- One side is plastic: stop, read plastic to metal instead.
Prep that actually changes the outcome
- Remove rust and paint down to a dull shine. A flap disc or 120-grit is enough; mirror polish is worse than a scratch pattern.
- Degrease with IPA or acetone on a clean rag. Change rags until one stays clean.
- On aluminum, do not touch the scuffed face. Skin oil is a release agent.
- Mix epoxy until it is one color. Streaks are unmixed resin and they stay liquid in the core.
- Clamp. Even a light clamp beats a “held for 30 seconds” joint on a 1-inch overlap.
Temperature: most consumer epoxies want roughly 15–25 °C. Below that they undercure and you blame the brand.
Galvanized, painted, and oily “new” metal
Galvanized steel in the matrix is not the same as bright mild steel. The zinc layer is soft and greasy. If you epoxy to the zinc, you have bonded to a weak coating; a hard knock shears zinc off the steel and the glue looks like it “failed.” Grind to parent metal on structural repairs, or use a construction adhesive rated for galvanized on non-structural flashing.
Powder coat and baked enamel are plastics sitting on metal. You are then in plastic-to-metal territory unless you grind through the coating. Threadlocker (anaerobic) is a third family: it cures in the absence of air inside a tight thread. It is the right product for a bolt, the wrong product for a lap joint you can see.
When glue is the wrong repair
Pressure vessels, steering, structural lifting points, gas fittings — weld, braze, or replace. Glue is for brackets, housings, magnets, tools, and sheet you cannot get a torch on. If you need the look of a weld bead, you still need a weld.
FAQ
Strongest glue for metal to metal?
Two-part structural epoxy on scuffed, degreased metal. Brand matters less than prep.
Does super glue work on metal?
On a tight, clean, low-peel joint, yes. It is not a substitute for epoxy on a bracket that takes impact.
Can glue replace welding?
Only for non-structural or inaccessible joints. It does not replace a weld on anything that can hurt someone if it lets go.
Stainless?
Abrade through the passive film, degrease twice, then epoxy. Skip PVA and silicone.