How do I mix and use two-part epoxy properly?

Epoxy is the only common adhesive that genuinely fills gaps at full strength and bonds to non-porous materials. It is also the one where a small error in mixing produces something that never hardens.

Safety

What you'll need

• two-part epoxy
• mixing cups and sticks
• nitrile gloves
• abrasive paper and degreaser
• clamps or tape (optional)

Step by step

1. Prepare the surfaces before mixing.Epoxy's pot life starts the moment the parts meet. Abrade both faces to a matt key, degrease with isopropanol or a proprietary cleaner, and have clamps and packing ready.
2. Check the mixing ratio and how it is measured.Ratios are given by volume or by weight and the two are not interchangeable. Some five-minute epoxies are 1:1, structural systems are frequently 5:1 or 2:1.
3. Measure accurately.Epoxy is a chemical reaction, not a drying process. Excess of either part does not cure and remains permanently soft, which is the commonest epoxy failure.
4. Mix thoroughly for a full minute.Scrape the sides and the bottom of the cup repeatedly. Unmixed resin clinging to the cup wall is exactly what ends up in the joint.
5. Transfer and mix again.Double-potting — pouring into a clean cup and mixing a second time — eliminates the unmixed film left on the first cup. It is standard practice in boatbuilding for good reason.
6. Understand pot life versus working time.The reaction is exothermic and heat accelerates it. A thick pot goes off far faster than a thin film, so spread it out or use a shallow tray to buy time.
7. Wet out both surfaces first.Apply a thin coat to each face and let it soak in on porous materials, then apply the thickened or full-strength mix and bring the parts together.
8. Thicken it if you need to fill a gap.Colloidal silica or a proprietary filler turns runny epoxy into a paste that stays where it is put and still cures at full strength.
9. Clamp lightly.Epoxy does not need pressure to bond, only contact. Heavy clamping squeezes it out of the gap it was supposed to fill. Tape or light clamps are usually enough.
10. Cure warm and leave it alone.Cure time roughly halves for every ten degrees warmer. In the cold it can take days, and below the minimum temperature it never reaches full strength.
11. Remove amine blush before recoating or finishing.Some epoxies leave a waxy film as they cure. Wash it off with warm soapy water and abrade before painting, or nothing will stick to it.

Tips

• Double-pot: mix, transfer to a clean cup, mix again. It removes the unmixed film that causes most soft-cure failures.
• Spread the mixed epoxy out in a shallow tray to slow the reaction and buy working time.
• Never clean epoxy off skin with solvent — it carries the resin through the skin. Wipe with a dry cloth, then wash with soap and water.

Common mistakes

• Estimating the ratio by eye — Epoxy cures by reaction between two components in a fixed proportion. Get it wrong and the excess never reacts, leaving a permanently soft, weak joint that cannot be rescued.
• Mixing quickly and going straight to the joint — The film clinging to the cup wall is unmixed and ends up in the joint. Scrape the sides and bottom, and transfer to a second cup.

If it doesn't work

Questions people ask