How to Paint a Sandwich Panel: Paint, Primer and Steps

Yes, you can paint a sandwich panel, but not like a wall. The panel’s outer face is galvanized steel sheet with a factory coating, and if the new paint isn’t applied properly, it will peel off after the first summer. Here’s when it’s worth doing, which paint to use, and the steps to make it last.

Does sandwich panel need to be painted?

It’s not mandatory. The panel comes factory-coated in the RAL color you choose, and that coating protects the sheet for many years (we explain this in how long a sandwich panel lasts). There are three cases where painting makes sense:

• Touch-ups: scratches, dents or cuts where the steel is exposed. These should be touched up quickly to prevent rust.
• Color change: you want a different color on an existing façade or roof.
• Renovation: the coating has lost its shine or has a white powdery film (chalking) after years in the sun.

If the panel is punctured, has a wet core, or the sheet is badly rusted, painting won’t fix it: those panels need to be replaced. And if you’re buying new panels, it’s far more cost-effective to order them already in the color you want: check the RAL color chart.

Which Paint to Use

Avoid synthetic (alkyd) enamels directly on galvanized surfaces: over time they lose adhesion and flake off. Always read the paint’s technical data sheet and check that it’s suitable for pre-coated galvanized sheet metal.

How to Paint a Sandwich Panel Step by Step

1. Choose the right day: no rain forecast, no dew, and mild temperatures (roughly between 10 and 30°C). In summer, don’t paint the roof in direct sunlight: the sheet gets very hot and the paint won’t dry properly.
2. Clean: use water with neutral soap and a soft brush, or a low-pressure power washer, avoiding joints and trims. Remove moss, dust and chalking.
3. Degrease: apply a degreaser compatible with the paint and rinse. Let it dry completely.
4. Sand: lightly sand (fine sandpaper or an abrasive pad) to remove the shine from the coating. Don’t sand down to the steel.
5. Treat the rust: at cuts, edges and dents, brush off any loose rust and apply anti-corrosive primer.
6. Prime: apply a coat of adhesion primer over the entire surface, allowing the manufacturer’s recommended drying time.
7. Paint: apply two thin coats of finish with a roller, using a brush for overlaps and trims, or an airless sprayer for large surfaces. Paint in the direction of the ribs.
8. Check joints and screws: take the opportunity to replace any damaged washers and seal with polyurethane sealant or neutral-cure silicone, never acidic silicone.

Painting the Roof: Safety

On a sandwich panel roof, only step on the purlins (the line of screws), never in the middle of the span or on the skylights. Work with a harness attached to a lifeline, and don’t go up if it’s windy or the sheet is wet. If the roof is high or large, hire a company with the proper safety equipment.

Can the Inner Face Be Painted?

Yes, and it’s easier: it’s not exposed to sun or rain. Clean it, sand lightly, prime it and apply water-based acrylic paint. In cold storage rooms or food industry settings, use paints suitable for that purpose or, better still, leave the factory coating as is, since it’s easy to clean.

Paint or Replace the Panel

For the rest of the maintenance (cleaning, inspections and what not to do), check out our sandwich panel roof maintenance guide. If you need new panels, take a look at the sandwich panel price per m².

Frequently asked questions