The primer (base or preliminary coat) is the most important yet invisible step before any paint job. Its purpose is to prepare the substrate for the topcoat: it consolidates chalking or sanding surfaces, seals highly absorbent areas such as fresh plaster or plasterboard and creates a uniform bonding layer. Without a primer the paint soaks in unevenly, covers poorly and flakes off more easily later.
Different products are used depending on the substrate. A deep primer consolidates highly absorbent mineral surfaces. A bonding primer is used on smooth, low-absorbency substrates such as old oil or latex paint. For water or nicotine stains an isolating blocking primer is needed so the stains do not bleed through. On floors, the primer binds the screed dust-free and improves the adhesion of levelling compound or adhesive.
The process is simple: clean the surface, remove loose material, then apply the primer with a roller, brush or floor brush and observe the drying time specified by the manufacturer (usually 4 to 12 hours). Only then does the topcoat or covering follow.
A primer is always needed when the substrate is new, highly absorbent, chalking, smooth or stained. On intact, already-painted walls in good condition it may sometimes be omitted. When in doubt a wipe test helps: if white dust sticks to your hand, priming is required.
Material costs for primer in Switzerland are usually around CHF 3 to 8 per m²; the labour is normally included in the painter's per-m² price. If billed separately, CHF 4 to 9 per m² including material is realistic (guide values, no guarantee).