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

• Provides adhesion: Primer is designed to bond to metal, plastic and filler and to give a suitable surface for basecoat and clearcoat to adhere to.
• Adds corrosion protection: On metal panels, the right primer helps keep moisture and salt away from the steel underneath, reducing the risk of rust starting under the paint.
• Levels the surface: High-build and surfacer primers fill fine scratches and minor lows so that, once sanded, the panel is smooth before colour is applied.
• Influences final finish: Poorly applied or badly sanded primer can print through into the topcoat as sinkage, edge-mapping or texture differences.

Where you’ll see it

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.

Written by Danny Argent. Last updated 21/11/2025 15:20