Primer improves adhesion, evens out porosity and can block certain stains. Whether you need it depends on what is already on the wall and what will go on top.
Use primer when the surface needs it
- New drywall, bare plaster or fresh filler is absorbent and often needs a suitable primer or mist coat.
- Glossy, very smooth or difficult surfaces may need an adhesion primer after proper cleaning and sanding.
- Smoke, water, grease or tannin stains may require a dedicated stain-blocking primer.
- A strong colour change can benefit from a tinted primer, depending on the topcoat instructions.
When it may be unnecessary
On a sound, clean, previously painted wall, many modern paints can be applied directly after cleaning, sanding damaged areas and spot-priming repairs. Do not add an extra product merely because it sounds safer; follow the paint system specified by the manufacturer.
How much primer should I buy?
Calculate the same paintable wall area as for topcoat, but usually use one coat. Primer coverage can be lower on porous surfaces, so use the coverage printed on the specific product. Spot priming reduces the quantity when only repairs need sealing.
Primer does not replace cleaning, scraping loose paint or fixing moisture problems. Paint over an active leak or damp wall and the finish is likely to fail.