First coat of paint or specially formulated liquid, used to provide a good key for subsequent coats of paint or as a protective base. It seals the surface and prevents paint from soaking into it.
A primer should always be used on all new unpainted surfaces-bo th externally and internally – that are to be painted with an oil-based paint: it is not normally necessary, however, when finishing with a water-based paint inside the house. Acrylic primer also acts as undercoat. All-surface primers are available, but it is preferable, whenever possible, to use the primers designed specifically for use on wood, metal, building boards, plaster or concrete.