Paint primers and undercoats are the first coat of paint you will need to put on metal, wood such as MDF, and walls if the surface has never been painted. Undercoat is traditionally a separate layer, formulated to provide the best surface for the desired topcoat; promoting good adhesion, depth of colour, flatness and longevity. Painting a base layer ensures that your chosen colour can be achieved in fewer coats. If you are covering a dark colour or a surface that's never been primed or painted, you will need to use an undercoat. It can also provide a better coverage and hiding power than just applying extra top coats.