80% of the final result of a paint job depends on how the surface is prepared, not on the tin of paint chosen. Skipping the primer or filler to save time is the most common reason a freshly painted wall looks uneven or loses colour in patches.
When primer is needed
Primer or sealer is always applied on new plaster, very porous walls or stained ones, with a coverage of 10 m² per litre (one coat), in 5-litre tins. If the wall already has paint in good condition without stains, you can save it and paint directly.
Worked example: to prime 35 m²: 35 ÷ 10 = 3.5 litres, rounded up to 1 tin of 5 litres. This is the same calculation the PresupuestoYa calculator applies automatically for the Painter trade.
How much filler it takes to fill cracks
Filler or patching paste fills cracks and holes and evens out the surface before painting, with a coverage of 1 kg per 2–3 m² in a thin application. If there are many cracks, multiply the estimated consumption by 1.5.
- Acrylic filler: more flexible, for cracks that move.
- Plaster filler: more rigid and sands better.
- For large irregularities, use bonding paste or a skim coat instead of filler.
Sanding: mandatory over gloss paint or old enamel
Over gloss paint or old enamel, sanding is mandatory for the new coat to adhere well: painting directly onto a very smooth surface usually ends in flaking within a few months.
Sealing joints with paintable silicone
Before painting, seal the joints and cracks where materials meet (wall–joinery, worktops) with paintable (acrylic) silicone, never pure sanitary silicone — the latter doesn't take paint and the bead stays visible. In bathrooms and kitchens, check and renew the sealing if it is yellowed or mouldy before repainting.
Frequently asked questions
Can I paint directly onto new plaster without primer? It isn't recommended: new plaster absorbs paint unevenly without a prior primer, giving an uneven finish.
How much filler do I need for a wall with many cracks? Multiply the standard consumption (1 kg per 2–3 m²) by 1.5 if the wall has many cracks or irregularities.
Which silicone do I use to seal joints that I'll paint afterwards? Paintable acrylic silicone, never pure sanitary — sanitary doesn't take paint and leaves the bead visible.
Conclusion
Investing in primer, filler and sanding before picking up the roller is what separates a professional paint finish from one that looks uneven within a few months. Quoting the preparation as its own item, not as a "free" step included in the painting, reflects the real time a job done well takes.
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