Primer

Primer is the first stage on any paint system. Both clear-over-base and solid colour styles of paintwork will have a primer on the metalwork before further layers of paint are applied at the factory. This creates a good bond to the metalwork or plastic while giving a suitable surface for subsequent paint layers to stick to.

What it means

Primer is the first proper paint layer used when repairing or refinishing a panel. After metalwork, plastic repair or filler work has been completed, primer is applied to seal the surface, help the next coats stick and start levelling out sanding marks. Different primers are used for different jobs - for example etch or epoxy primers for bare metal, adhesion-promoting primers for plastics and high-build primers for filling minor imperfections before flatting and painting.

Why it matters

Adhesion is the primary job. Primer is formulated to bond to metal, plastic and filler while presenting a surface that basecoat and clearcoat will hold onto, and everything above it depends on that grip.

On metal panels it also buys corrosion protection, keeping moisture and salt away from the steel and reducing the chance of rust starting underneath the paint where nobody can see it begin. High-build and surfacer primers do a third job, filling fine scratches and minor lows so that once sanded the panel is genuinely flat before colour goes anywhere near it.

Get any of that wrong and it shows through. Poorly applied or badly sanded primer prints into the topcoat as sinkage, edge-mapping or a texture that does not match the panel beside it.

Which primers do which job

You will see primer mentioned on bodyshop estimates, technical data sheets and repair instructions - for example etch prime bare metal, apply epoxy primer, high-build prime and flat before basecoat. Inspectors and detailers may talk about damage down to primer or previous repairs where primer edges are starting to show through the topcoat.

Context

Primer sits between the bare substrate and the visible paintwork. In a typical repair, the sequence is bare metal or plastic, then suitable primer, then basecoat (colour) and clearcoat. There are many primer types, including etch primers, epoxy primers, high-build surfacers and wet-on-wet primers, each chosen to suit the substrate, repair size and paint system. When polishing or correcting paint, you should not be cutting into primer - if it is visible, the top layers have already been compromised and the discussion usually moves towards repair and repaint rather than further correction.

Common mistakes

• Applying colour or clearcoat directly over bare metal or filler without suitable primer, leading to poor adhesion and early failure.
• Using the wrong primer type for the substrate, such as standard primer on plastics that really need a dedicated adhesion promoter.
• Leaving heavy sanding marks or edges in the primer and expecting the topcoat to hide them, which often leads to sinkage and visible repair lines later.
• Building primer too thick or rushing flash-off and curing times, trapping solvents that later cause die back, solvent migration or mapping through the topcoat.

Authored by Danny Argent. Reviewed by Gary Wray. Last updated 16 May 2026.