Repair guide
How to attach a mild-steel plaque to a concrete wall
MS polymer is the right adhesive for steel on concrete outdoors, because it bonds to both, stays flexible enough to absorb the very different thermal movement of the two materials, and is unaffected by rain and UV.
The short answer
Use an MS polymer adhesive. Steel and concrete expand at noticeably different rates in the sun, so a rigid adhesive concentrates that movement at the bond line and eventually cracks it. MS polymer flexes instead. Brush the concrete clean and dry, abrade the back of the plaque, apply the adhesive in vertical beads rather than a solid bed so trapped water can drain, and support the plaque with tape until it cures.
- Difficulty
- Moderate
- Time needed
- 20 minutes work, 24 to 48 hours cure
- What you need
- MS polymer adhesive and a caulking gun, stiff brush, 80 grit abrasive paper, isopropyl alcohol, spirit level, strong masking tape or a prop, pencil
Outdoor steel-on-masonry fixings fail for three reasons, and the adhesive is only one of them. The first is dust: concrete and render shed a fine powder that a brush removes and a cloth just moves around, and adhesive bonded to dust lifts off cleanly. The second is water. A plaque bedded solidly on a wall traps water behind it, and behind a steel plaque that means rust streaks down the render within a season. The third is movement, which is where the choice of adhesive comes in.
MS polymer, sold in the UK under names like Stixall and Gun a Nail, sits between a sealant and an adhesive. It grabs quickly, holds a decent weight without slumping, cures with moisture from the air, and stays slightly rubbery for years outdoors. That flexibility is what lets a steel plate and a concrete wall move at different rates without tearing the joint.
A dry wall matters more than most people expect. MS polymer needs moisture in the air to cure, but bonding onto a visibly damp or freshly rained-on surface gives a weak interface. Pick a dry day and give the wall a couple of hours.
Step by step
- Check the wall is sound. Tap it, and if the render sounds hollow or crumbles under a thumbnail, fix that first or fix through to something solid.
- Brush the area with a stiff brush to remove dust and loose grit, then leave it to dry fully.
- Abrade the back of the plaque with 80 grit to bright metal and wipe it with isopropyl alcohol.
- Hold the plaque up and mark its position lightly with a pencil and a spirit level.
- Apply MS polymer to the back of the plaque in vertical beads about 40mm apart, keeping 15mm back from the edges.
- Press the plaque onto the wall firmly, then pull it off, wait two minutes, and press it back. That open time considerably improves the grab.
- Tape it in place, or prop it, and leave the support on for 24 hours.
- Leave 48 hours before it sees heavy rain if you can. Full cure through a thick bead takes longer than the surface suggests.
What to avoid
Do not bed the plaque in a solid layer of adhesive. Water gets behind it, has nowhere to go, and rusts the plaque from the back.
Do not use cyanoacrylate or any rigid adhesive. Concrete is porous and alkaline, steel and concrete move at different rates, and a rigid line between them cracks.
Do not fix onto blown render or flaking paint. The adhesive will outlast the surface it is stuck to.
A heavy plaque above head height should be mechanically fixed with plugs and screws, with adhesive only as a bedding and seal. Uncured MS polymer is difficult to remove from brick and render, so mask the area if the finish matters.
The full bonding guide
Everything about gluing Mild steel to Concrete
Last reviewed 13 September 2026.