Repair guide
How to bond a fibreglass panel to a steel frame
Two-part epoxy gives the strongest bond between GRP and steel, but the two materials expand at very different rates, so a long panel needs either a flexible adhesive or a bond line that can take the movement.
The short answer
Abrade the steel to bright metal, sand the gel coat off the GRP bonding face, degrease both and bond with a structural two-part epoxy over a generous overlap. The catch is thermal movement: GRP and steel expand at noticeably different rates, so on a panel longer than about a metre, or anything that sits in the sun, use a flexible polyurethane or MS polymer adhesive instead and accept the lower ultimate strength.
- Difficulty
- Difficult
- Time needed
- 1 hour work, 24 hours cure, 7 days to full strength
- What you need
- Structural two-part epoxy or polyurethane panel adhesive, 80 grit abrasive or flap disc, isopropyl alcohol, vacuum, clamps or straps, primer for exposed steel, gloves, dust mask, eye protection
This is a common repair on vehicle bodies, enclosures and boats, and it fails for one of two reasons.
The first is preparation. GRP's outer gel coat is smooth, often waxy from the mould, and it is not the strongest part of the laminate. Bonding to unsanded gel coat gives a joint that peels the gel coat off the glass underneath. Sand through the sheen to expose the laminate, and on the steel side abrade to bright metal and bond within the hour.
The second is movement. Steel expands roughly half as much as a typical GRP laminate for the same temperature change. Over a 200mm bracket that difference is irrelevant. Over a two metre panel in direct sun it is several millimetres, and a rigid epoxy line has to absorb all of it. That is why panel bonding on vehicles is done with flexible polyurethane adhesives rather than epoxy: they stretch.
So pick by size and exposure. A small, shaded, structural bond takes epoxy. A long, exposed panel takes polyurethane or MS polymer, with a deliberately thicker bond line, because a thicker flexible layer absorbs more movement.
Galvanic corrosion is worth a thought too. Bare steel against a damp laminate rusts, and rust under a bond line lifts it. Prime the steel outside the bond area and seal the joint edges.
Step by step
- Dry fit the panel and mark the bond area on both parts.
- Sand the GRP bonding face with 80 grit until the gel coat sheen is gone and the laminate texture shows.
- Abrade the steel to bright metal with 80 grit or a flap disc.
- Vacuum the dust, then degrease both faces with isopropyl alcohol and do not touch them again.
- Decide on the adhesive: structural epoxy for a short shaded bond, flexible polyurethane for a long or sunlit panel.
- Mix and spread an even layer on both faces, or run beads if you are using a flexible adhesive and want a thicker bond line.
- Bring the parts together, work the panel down to expel air, and clamp or strap evenly.
- Tool the squeeze out into a fillet at the edges, leave 24 hours clamped, and prime or paint any exposed steel afterwards.
What to avoid
Do not bond to unsanded gel coat. It separates from the laminate under load.
Do not use rigid epoxy on a long panel that sees sun. Thermal movement will crack the bond line.
Do not leave bare steel exposed at the joint edge. Rust creeps under the bond.
Sanding GRP produces glass dust that irritates skin, eyes and lungs, so wear a proper mask, goggles and long sleeves. A panel that is structural or safety-related on a vehicle or boat should be bonded to the panel manufacturer's specification, not a general guide.
The full bonding guide
Everything about gluing GRP / fibreglass to Mild steel
Last reviewed 13 September 2026.