Epoxy forms strong, lasting bonds with most metals, wood, concrete, ceramics, glass, and fiber-reinforced composites. It struggles with a specific group of low-energy plastics, including polyethylene, polypropylene, and Teflon. Understanding which materials fall into each category, and why, can save you from a failed joint or a wasted project.
Why Epoxy Sticks So Well to Most Surfaces
Epoxy’s chemistry gives it a natural advantage over most other adhesives. The resin contains hydroxyl groups that form hydrogen bonds with surfaces that have their own hydroxyl groups, which includes metals with oxide layers, glass, ceramics, and wood. These hydrogen bonds are the primary driver of adhesion, but a secondary interaction between the hydroxyl groups on a surface and the benzene rings in the epoxy backbone adds meaningful extra grip. Together, these chemical forces create bonds that go beyond simple “stickiness” and approach structural strength.
On rough or porous surfaces like wood and concrete, epoxy also flows into microscopic crevices and hardens there, creating a mechanical interlock. This combination of chemical bonding and physical anchoring is what makes epoxy one of the most versatile structural adhesives available.
Metals
Epoxy bonds exceptionally well to steel, aluminum, copper, brass, and most other common metals. Metal surfaces naturally form thin oxide layers that are rich in hydroxyl groups, giving epoxy plenty of chemistry to grab onto. Aluminum and steel are the most thoroughly tested substrates for epoxy adhesion. The standard industrial test for measuring adhesive shear strength (ASTM D1002) uses metal as its reference substrate, which reflects how reliably epoxy performs on these surfaces.
The key to a strong metal bond is surface preparation. Lightly sanding with 80 to 120 grit paper removes loose oxide, grease, and mill scale while creating microscopic texture for the epoxy to grip. After sanding, a solvent wipe removes residual contamination. A quick test: if water beads up on the surface, it’s still contaminated. Wipe again with solvent and re-sand until water sheets evenly across the metal.
One important consideration with metals is thermal expansion. Metals and epoxy expand at very different rates when heated or cooled. Aluminum expands at roughly 25 parts per million per degree Celsius, while a typical epoxy can expand at around 75 parts per million. That mismatch creates internal stress at the bond line during temperature swings. In testing of bonded aluminum-composite joints, audible cracking was observed at sub-ambient temperatures, and CT scans confirmed tensile failures within the epoxy layer. For joints that will see wide temperature swings, using a slightly flexible epoxy formulation or designing the joint to minimize stress concentration helps prevent this kind of failure.
Wood
Wood is one of the best substrates for epoxy bonding. Its porous, fibrous structure lets liquid epoxy wick into the grain before curing, creating both a chemical bond and deep mechanical interlock. Hardwoods and softwoods both bond well, though dry wood performs better than wet wood since moisture in the grain can interfere with curing.
Sanding with 80 to 120 grit opens up the grain and removes any surface finish or oxidized wood fibers. Finer grits can actually reduce bond strength by polishing the surface too smooth. Oily tropical hardwoods like teak and rosewood need a solvent wipe before bonding, since their natural oils create a barrier that weakens adhesion.
Concrete, Stone, and Masonry
Epoxy bonds strongly to concrete, brick, granite, marble, and most natural stone. These materials are rich in silica and metal oxides, both of which provide hydroxyl-rich surfaces for chemical bonding. The porosity of concrete and unpolished stone also allows epoxy to penetrate and lock in mechanically.
On concrete, the surface should be clean, dry, and free of dust or curing compounds. Polished stone like granite countertops will bond better after light sanding with 80 to 100 grit to break the polished surface and give the epoxy something to grip.
Fiberglass, Carbon Fiber, and Aramid Composites
Epoxy is the standard matrix resin for high-performance composites, and it bonds to reinforcement fibers better than polyester or vinyl ester alternatives. Glass fiber, carbon fiber (graphite), and aramid fiber all form strong bonds with epoxy, which is why carbon-aramid fiber-reinforced epoxy laminates are used extensively in aerospace and automotive applications. The high strength-to-weight ratio of these composites depends entirely on the epoxy maintaining a reliable bond with each individual fiber.
When bonding to the surface of an already-cured composite, sanding with 80 to 120 grit and solvent wiping is the standard approach. Avoid sanding so aggressively that you cut through fibers, since the goal is to rough up the resin surface, not damage the reinforcement underneath.
Glass and Ceramics
Glass and ceramic surfaces are hydroxyl-rich, making them excellent candidates for epoxy bonding. Clean glass bonds well without sanding, though a light scuff with fine abrasive gives extra insurance on structural joints. Ceramics like porcelain and alumina bond reliably after cleaning with solvent to remove any surface oils.
Materials Epoxy Will Not Bond To
Epoxy fails on a specific family of materials that share one thing in common: low surface energy. These surfaces are so chemically inert that epoxy can’t form hydrogen bonds with them, and they’re typically too smooth for mechanical interlocking. The list includes:
- Polyethylene (including HDPE), used in plastic bags, milk jugs, and cutting boards
- Polypropylene, found in food containers and plastic furniture
- Teflon (PTFE), the nonstick coating on cookware
- Silicone, used in molds, sealants, and bakeware
- Nylon, which bonds poorly without special surface treatment
- Mylar, a polyester film used in packaging and electronics
PVC, MDF, and melamine-coated surfaces also resist epoxy adhesion. This is actually useful information in both directions: if you need to make a mold for epoxy casting, polyethylene, silicone, and melamine all work as release surfaces since cured epoxy will pop right off.
Surface Preparation Makes or Breaks the Bond
Even on materials epoxy bonds well with, a dirty or contaminated surface will cause failure. The general preparation sequence is the same regardless of substrate: clean off grease and oil, abrade the surface, clean again, then apply epoxy before dust or moisture can settle.
For sanding, 80 to 100 grit is sufficient for most bonding applications and for surfaces that will receive a primer or filler. Going finer than 400 grit can actually hurt adhesion because the surface becomes too smooth for the epoxy to grip. After sanding, test for contamination by wetting the surface with water. If the water beads up rather than spreading into an even film, residual oil or grease is still present and needs another solvent wipe.
On porous materials like wood and concrete, applying a thin “seal coat” of unthickened epoxy before the structural bond allows the resin to soak into the substrate. Once that seal coat begins to gel, applying the thickened bonding layer on top creates a stronger joint than a single thick application, since the epoxy has penetrated deeper into the material.
Temperature and Long-Term Durability
Thermal expansion mismatch is the most common cause of epoxy bond failure over time, especially when joining two different materials. Every material expands and contracts at its own rate as temperature changes. When the substrate expands faster or slower than the cured epoxy, stress builds at the bond line. Research on bonded composite-to-aluminum joints found that the epoxy layer (expanding at roughly 75 parts per million per degree Celsius) is under significant tension when temperatures drop, since both the composite and aluminum expand far less. Over repeated thermal cycles, these stresses can initiate cracks that eventually lead to full adhesive failure.
For projects exposed to temperature extremes, choosing an epoxy formulated with some flexibility helps it absorb differential movement. Keeping bond lines thin also reduces the total stress, since a thinner layer of epoxy has less volume to expand and contract. Joints between similar materials (metal to metal, or composite to composite) naturally experience less thermal stress than joints between dissimilar materials.