Primer Calculator
Works out primer, sealer or mist coat. Pick the substrate — what you are priming changes both the coverage and which product will actually work.
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The formula
litres = area ÷ coverage per litre × coats
Coverage is the whole calculation, and it is the figure tins are most optimistic about. The label quotes a smooth, sealed, previously painted surface; bare plaster, textured render and a strong colour change all take noticeably more.
What to watch out for
Primer is not one product, and using the wrong one is worse than using none:
- New plaster wants a mist coat, not primer — emulsion thinned about 70:30 with water, so it soaks in and keys. Full-strength paint on fresh plaster peels off in sheets later.
- Stains need solvent or shellac. Water-based stain block over nicotine or a water mark reactivates the stain and it comes straight back through.
- Metal primers are metal-specific. Steel, aluminium and galvanised each want a different one.
- Knots need knotting solution before primer, or resin bleeds through the finish within a year.
Frequently Asked Questions
Do I need to prime new plaster?
Not with a proprietary primer — with a mist coat, which is ordinary emulsion thinned roughly 70:30 with water. It soaks into the plaster and gives the topcoat something to grip. Undiluted paint on fresh plaster peels later.
How much primer do I need?
About 9 m² per litre for general primer, so a 40 m² area takes around 4.5 litres for one coat. Masonry stabiliser soaks in far more — closer to 5 m²/L.
Can I use emulsion as primer?
On new plaster, yes — thinned as a mist coat, that is the correct approach. On bare wood, metal or stained surfaces, no: those need a primer formulated for the substrate.