Primer, Undercoat or Mist Coat: What Is the Difference?

Walk into any paint shop and the shelf is full of products that sound the same. They are not. Each one has a job.

Primer

Primer is the first layer on a raw or difficult surface. Its job is to seal and to grip. It gives the paint above it something to hold onto and stops the surface soaking up your topcoat.

Primers are designed for specific materials. There are primers for bare wood, metal, glossy surfaces, stained surfaces and masonry. Using the wrong one is a common cause of peeling.

Undercoat

Undercoat sits between primer and gloss on woodwork and metal. It builds thickness and a flat, even base for the finish coat, and it helps the colour show true.

Many modern paints skip the undercoat, using a combined primer-undercoat instead. In practice, on bare or heavily repaired wood, a dedicated primer followed by an undercoat still gives the best result.

Mist coat

A mist coat is a thinned emulsion used on new plaster. It seals the surface without sitting on top of it. We cover it in detail in painting new plaster.

A mist coat is not a primer in the technical sense, but it does a similar job on a very specific surface.

Which do you need?

• New plaster: Mist coat, then emulsion
• Previously painted wall in good order: Often no primer; a clean and a light sand
• Bare wood: Wood primer, undercoat, then topcoat
• Knots in wood: Knotting solution first, to stop resin bleeding
• Glossy painted woodwork: Sand, then adhesion primer
• Stained wall or ceiling: Stain-blocking primer
• Bare metal: Metal primer to prevent rust
• Masonry or render outside: Masonry sealer, then masonry paint

The idea is simple: match the primer to the surface.

When you can skip it

On a sound, clean, previously painted wall, a good emulsion over a light sand and wipe-down often needs nothing more. Skipping primer is fine when the surface is already in good order. It is not fine on raw, stained or repaired areas.

Can paint-and-primer in one do it all?

For a colour change on sound walls, yes, they can help with coverage. They do not replace a proper primer on bare plaster, stains or shiny woodwork.

Preparation still comes first

No product saves a dirty, loose or glossy surface. Our guide to wall preparation sets out what comes before the first coat. For woodwork, see painting skirting boards, doors and woodwork.

If you would rather leave the choosing to someone who does it daily, our interior decorating team will specify the right system for each surface. Ask us for a quote.