What is the difference between primer, undercoat and topcoat?

These three get treated as three coats of roughly the same thing, and they are doing three genuinely different jobs. Knowing which lets you skip the ones you do not need.

Step by step

1. Understand what primer does.It bonds to a difficult or absorbent surface and gives the following coats something to grip. It also seals porosity, so a bare surface does not drink the topcoat unevenly.
2. Understand what undercoat does.It builds a uniform, opaque base in a colour close to the topcoat. It hides what is underneath and lets you get even colour in fewer topcoats. It is not designed to be durable on its own.
3. Understand what topcoat does.Colour, sheen and durability. It is formulated to resist washing, knocks and light. It is not formulated to stick to bare or slick surfaces, which is why primer exists.
4. Prime whenever the surface is bare or difficult.New plaster, bare timber, metal, plastic, tile, or any repaired patch. Also anywhere the existing finish is glossy and cannot be keyed enough by sanding alone.
5. Skip primer over sound existing paint.If the current paint is in good condition and you have keyed it, primer adds little. Repainting a wall the same colour is often just two topcoats.
6. Use undercoat when the colour change is large.Dark to light, or a strong colour to a pale one. Undercoat tinted towards the new colour saves a topcoat or two and gives a more even result.
7. Skip undercoat for small colour changes.Similar shades on sound paint rarely need it. Two topcoats will cover.
8. Know what a mist coat is.For new plaster: emulsion thinned with around 10 to 20% water as a first coat. It soaks in and seals the surface. Full-strength paint straight onto new plaster peels off later.
9. Be realistic about all-in-one products.Primer-undercoats work well on moderately difficult surfaces and save time. They are not as good as a dedicated adhesion primer on tile, laminate or foil, and not as opaque as a proper undercoat over a strong colour.
10. Match the products to each other.Water-based over water-based, solvent over solvent, unless you have specifically used an adhesion primer to bridge them. Mixing systems without one is a common cause of peeling.
11. Check what your topcoat expects.Many modern paints state their own system on the tin — some need no undercoat at all, others specify a particular primer. Following that beats general advice.
12. Do not use undercoat as a topcoat.It is matt, absorbent and not durable. Left uncoated it marks immediately and cannot be cleaned.

Tips

• Primer for adhesion, undercoat for colour, topcoat for wear. Decide which of those you actually need.
• New plaster wants a thinned mist coat first. Full-strength paint straight on will peel later.
• Read the topcoat tin. Modern paints often specify their own system and it overrides general rules.

Common mistakes

• Using undercoat as a finish — It is absorbent and not durable by design. It marks immediately and cannot be washed.
• Painting new plaster at full strength — The plaster absorbs unevenly and the paint film does not bond properly, so it peels later. A thinned mist coat seals it first.

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