Paint and Surface Treatment

Interior and exterior · Primers, coverage and coats · Choosing the right paint type

Good painting is about more than colour. The choice of paint type, number of coats, primer and correct surface preparation determines how long the result lasts. This guide gives you the full picture — from calculating how much you need to avoiding the most common mistakes.

Paint types — interior

Acrylic paint (water-based)

• By far the most common paint type for interior walls and ceilings
• Dries quickly (2–4 hours between coats), low odour, cleans up with water
• Available in a range of sheen levels. Exact gloss numbers and naming conventions vary between manufacturers and product ranges
• Rule of thumb: Lower sheen is often used on ceilings and in living spaces, while higher sheen is often chosen where a tougher, more washable finish is wanted. Always follow the product's intended use

Alkyd emulsion (universal paint)

• Water-based with alkyd resin — excellent coverage and sheen, dries slightly slower than pure acrylic
• Suitable for doors, window frames and panelling where a harder, more abrasion-resistant finish is desired

Primer / sealer

• Required when: new plasterboard, highly absorbent surfaces (masonry, bare timber), changing from a dark to a light colour, stained surfaces
• Primer seals the substrate and can improve adhesion, even out absorbency and make topcoat performance more predictable. The effect on total consumption varies by system
• Typical coverage: 8–12 m²/L — higher than topcoat because it is thinner

Paint types — exterior

Exterior acrylic paint

• Many exterior paints for timber cladding are vapour-permeable and designed to handle moisture movement. Properties vary between products and systems
• Typical coverage is often around 4–8 m²/L, but this varies with product, substrate, application method and surface texture
• Recoat time, minimum application temperature and weather limitations are product-dependent. Always follow the technical data sheet and avoid hot surfaces or conditions the manufacturer advises against

Wood stain / preservative

• Opaque stain: forms a film on the surface, good colour coverage, but can peel over time
• Transparent stain: penetrates the timber, shows the wood grain, easier to maintain
• New pressure-treated timber must dry and be ready for coating in accordance with the product manufacturer's recommendations. The required waiting time varies with product, moisture content, weather and coating system

Layer build-up

Calculating coverage and coats

Paint consumption is calculated as: L = (Area × Number of coats) ÷ Coverage rate (m²/L)

Example: 30 m² wall, 2 coats, coverage 8 m²/L → (30 × 2) ÷ 8 = 7.5 L. Round up according to the pack sizes available for the chosen product

• Absorbent surfaces (new plasterboard, masonry, bare timber): use the lower coverage figure from the product data sheet
• Smooth surfaces (previously painted): use the higher figure
• Always add 10% extra for edges, spillage and residue in the tin

Step-by-step guide

Step 1 — Surface preparation

A good result starts with proper preparation — this is where most DIYers go wrong by jumping straight to painting.

• Scrape off loose, flaking paint with a scraper — and avoid dry-sanding or heat-stripping until you have ruled out lead (see warning above)
• Fill loose joints and holes with filler; sand with 80–120 grit after drying
• Wash greasy or dirty surfaces with sugar soap or a degreaser
• Mask floors, skirting boards and windows with painter's tape and plastic sheeting

Step 2 — Primer (for new surfaces)

• Apply primer with a suitable roller or brush in accordance with the manufacturer's recommendations for the substrate
• Allow to dry in accordance with the manufacturer's instructions before recoating
• Lightly sand with 180 grit after drying — removes nibs and improves adhesion for the topcoat

Step 3 — First coat

• Cut in at edges and corners with a brush (15–20 mm in from the edge)
• Roll in a W pattern, then level out vertically — avoid visible roller tracks
• A thin, even coat is better than a single thick one — a thick coat sags and leaves ridges

Step 4 — Second coat (topcoat)

• Wait until the first coat is completely dry (min. 4 hours indoors, 24 hours outdoors)
• Lightly sand with 180–220 grit between coats on walls for the best result
• Apply the second coat evenly using the method recommended for the product. Work systematically to avoid lap marks and visible transitions

Reference table

Common mistakes

• ✗Skipping primer on new plasterboard — causes flashing, visible joints and uneven topcoat
• ✗Painting over a damp or cold surface — the paint fails to bond and may start peeling within a year
• ✗Too thick a coat — runs, ridges and longer drying time; two thin coats always outperform one thick one
• ✗Sheen level too high on ceilings and raking surfaces — highlights every imperfection and tool mark
• ✗Stopping stirring during application — pigment settles and you end up with uneven colour
• ✗Painting outdoors in direct sun or below +5 °C — the film dries too fast or never properly cures
• ✗Forgetting to mask floors and skirting boards — most splatter accidents happen in the first few minutes

Calculate your materials

Use the calculator to find the correct number of litres and tins: