Evidence grade: A1 — First-hand testing (TDS/spec sheets) / B1 — Trade practice Effort: Easy · 30–60 min plus cure time · degreaser/solvent, sandpaper 60–120 grit, clean cloths, nitrile gloves, dust mask
Short answer: the bond is decided before you mix the epoxy. Degrease, abrade, degrease again, bond promptly. When an epoxy joint on metal fails, it is almost never the glue's fault — inadequate surface preparation is the usual suspect, and bond-failure analysis backs that up [1].
Why prep decides everything: the science
Adhesion is molecular contact. An adhesive must first wet the surface — flow out into intimate, molecule-to-molecule contact — because bond strength comes from weak intermolecular forces accumulating over a large contact area. Surface energy governs whether a liquid spreads or beads; contamination (oil, grease, dust, loose oxide) puts a weak layer between adhesive and metal, and the glue ends up bonded to the oil, not the steel [2].
Two failure modes tell the story [1]: - Adhesive failure — the glue peels cleanly off the metal. This points at the surface: contamination, weak oxide, or no prep. - Cohesive failure — the glue itself breaks. This points at the adhesive choice or the cure.
The practical rule from the trade: choose the right adhesive for a receptive substrate and the failure will "invariably" be a prep failure [1].
Abrasion does two jobs at once. It strips the weak layer (oxide, scale, old coatings) and it multiplies the contact area — fine micro-scratches can yield up to 40% additional surface area, which measurably improves adhesion [2]. For metals, guidance converges on a uniform matte profile roughly equivalent to sandpaper abrasion (one trade guide cites 150–250 microinches RMS for metals) [1]. A useful subtlety: on aluminum, abrasion chemically changes what you bond to — aluminum re-oxidizes within minutes, so the surface you're actually gluing is fresh aluminum oxide [2]. Prep it and bond it the same day.
The sequence
1. Degrease first. Remove oil, grease, dirt, and cutting fluid with a solvent — isopropyl alcohol for light contamination; mineral spirits or naphtha for heavy oils — then follow with a water/IPA wipe to be sure no residue or film remains [2]. Degreasing comes before sanding for a reason: sanding through an oil film just drives it into the surface.
2. Abrade. Sandpaper (60–120 grit for epoxy on metal), wire brush, or sandblasting for larger parts. Remove loose rust, mill scale, and shiny spots until the bond area is uniformly matte. Clean before and after abrasion [1].
3. Degrease again. Wipe the abrasion dust off with solvent and let the surface dry completely. Moisture interferes with cure; dust is just new contamination [1][2].
4. Bond promptly. Freshly abraded steel starts re-oxidizing immediately. Don't prep on Saturday and glue on Sunday.
5. Clamp with firm, even pressure. Contact is the whole game — firm, consistent pressure maximizes it, and bond strength generally increases with pressure, time, and warmth, up to a point [2].
For the chemistry of why epoxies grip metal at all — and what "cure" actually means — see Repair Lab B1. For why the same sequence fails on PE and PP (it's not the prep; it's the plastic), see Repair Lab B3.
⚠️ Stop rules
- Fuel tanks, pressure vessels, brake, steering, and suspension parts — Danger. DIY epoxy has no place on anything whose failure injures people. This is the same stop list as Repair Lab A4; it applies to adhesives too.
- Coated or galvanized metal: epoxy bonds to whatever is on top. If the bond area still carries paint, zinc, or plating, your joint is only as strong as that coating's grip — strip to bare metal in the bond zone.
- Parts that live in oil or heat: standard epoxy softens with temperature and hates oily service. If the part runs hot or stays greasy, the adhesive choice — not the prep — is wrong.
- Amine sensitization is cumulative. Uncured epoxy hardeners can trigger allergic sensitization that never goes away; once sensitized, even brief exposure causes reactions. See Repair Lab C4 and treat skin contact as a real hazard, not a nuisance.
- Exotherm on large masses. Pouring a big batch of mixed epoxy can run away thermally — see Repair Lab B8 before mixing more than a small repair needs.
PPE and ventilation
- Solvents: ventilate; nitrile gloves; eye protection. Keep solvents and solvent rags away from any ignition source — no open flames.
- Abrasion: dust mask or respirator, safety glasses, gloves. Metal dust is not breathing air.
- Old painted metal: assume lead paint until tested. If the coating predates the late 1970s, don't dry-sand it — test first or hand the job to someone equipped for it.
- Uncured epoxy: gloves always; ventilation; see Repair Lab C4 for the full sensitization picture.
Honest limits
- The 40%-surface-area and RMS figures are manufacturer guidance for their systems — directionally true across epoxies, not a guarantee for any specific product.
- Prep cannot fix a wrong adhesive choice, a contaminated-by-design surface, or a joint geometry that loads the glue in peel.
- Epoxy bond strength falls with temperature; check the product's service-temperature rating before bonding anything that runs warm.
Sources
- Master Bond — "Proper Surface Preparation for Adhesive Bonding" (tech white paper; University of Virginia mirror) — https://fpg.phys.virginia.edu/fpgweb/useful_info/Master_Bond-Proper_Surface_Preparation_Bonding.pdf — degrease, then clean before and after abrasion; adhesive vs. cohesive failure; "invariably" prep-failure rule; 150–250 microinch RMS guidance for metals; dry completely before bonding.
- Engineering.com (3M-sponsored) — "Smart Surface Preparation Makes Perfect Adhesive Bonds" — https://www.engineering.com/smart-surface-preparation-makes-perfect-adhesive-bonds/ — surface energy and wet-out; 50:50 water/IPA baseline; degreasers for heavy oils; ~40% additional surface area from micro-scratches; aluminum re-oxidation; firm pressure.
- Loctite "Product Application Guide" — dropped: no stable official URL could be verified (search surfaced only third-party spam mirrors, e.g. jra.jacksonms.gov and vaccination.gov.ng uploads — not citable as Loctite). The claims it was meant to support (clean, dry, grease-free surfaces; roughen smooth metals; keep oils away; wipe after abrasion) are confirmed by opened sources [1] and [2] and now cite them.
The prep sequence above is what a 1:1 structural metal-repair epoxy's bond assumes — skip a step and the bond is the first to know.
Repair Lab · Release 1 · Evidence last checked September 30, 2026 · A project of AOJEL