| Poor plaster, uneven blockwork, > 5 mm deviations | Re-plaster / level with screed mix, then skim, putty, prime, paint | Full build-up |
Skim coat is a thin finishing layer, 2 to 5 mm, whose only job is trueness of surface, not strength. It can be cement-based (mixed with fine, sieved plaster sand) or gypsum-based (the pre-mixed bags sold as "gypsum skim" or "wall skimming compound"). Gypsum gives a smoother, whiter finish and sets faster; cement skim is cheaper per square metre and more forgiving on damp-prone walls.
Putty is a fine paste used to fill pinholes and hairline marks after the skim has been sanded. It is applied in thin scrapes with a wide flex knife, allowed to dry, and sanded flush. Putty is not a levelling material, anything deeper than 1 to 2 mm should be handled in the skim stage, not buried in putty.
Direct paint, primer plus topcoats straight onto sound plaster, is a false economy on anything but the best machine-finished walls. Hand-applied plaster almost always carries ripples that two coats of paint will emphasise rather than hide.
If you are unsure where your walls fall, the article on skimming versus plastering draws the line in detail, and types of wall finishes compares the materials themselves.
Step 3: Prepare and Prime
Whatever the system, the preparation is the same discipline:
- Remove loose material. Scrape, then wire-brush flaking paint, loose plaster, and dust. Wipe the wall down with a damp sponge and let it dry.
- Repair first. Fill cracks (cut a V-groove along them first so the filler has a key), patch hollows, and re-plaster damaged corners. Let repairs cure fully before coating over them.
- Key smooth surfaces. Concrete cast against smooth formwork needs hacking, sanding, or a bonding primer before cement-based coats will hold.
- Prime. A primer does two jobs: it seals the wall's suction so the next coat dries evenly, and it bonds the layers above to the surface. On new plaster, use a specialist sealer or a thinned first coat of your emulsion as directed by the paint manufacturer. Skipping primer is the most common cause of patchy, uneven sheen on finished walls.
Primer deserves its own line in your budget, charged against the full wall area. It is the coat whose absence cannot be seen on completion day and cannot be hidden six months later.
Step 4: Apply the Skim Coat
Mix skim material to a trowel-consistency paste, stiff enough to hold on an inverted trowel, wet enough to spread without tearing. Work in sections you can finish in one pass:
- Load, then flatten. Pick up material with a steel trowel held at a low angle and spread it across the section with firm, overlapping strokes.
- Reduce the angle. As the coat firms up, go back over it with the trowel at a shallower angle to shave off ridges and fill hollows. Two thinner passes beat one thick pass, thick applications shrink, crack, and delaminate.
- Feather the edges. Leave no hard ridge at the boundary of each section; the next section should knit into it invisibly.
- Keep the tools clean. Skim that has started to set in your bucket or on your trowel contaminates fresh material and leaves hard streaks in the finish.
Cement skims need damp curing for the first 2 to 3 days in hot weather; gypsum skims simply need to dry through, typically 24 to 48 hours depending on ventilation. If a gypsum batch starts setting in the bucket mid-job, discard it, re-tempering set gypsum with water produces a coat that turns chalky and weak.
Step 5: Sand, Dust, and Putty
When the skim is bone-dry, sand it, but not randomly. Work through grits: a medium grit (100 to 120) to knock down ridges, then a fine grit (180 to 240) for the final pass. Use a sanding block or pole sander with a flat pad; loose paper in the hand gouges the surface.
The step almost everyone skips is dust removal. Sanding dust left on the wall prevents putty and primer from bonding, and it turns the first paint coat into a streaky mess. Vacuum or wipe the walls with a barely-damp sponge and let them dry.
Then putty: scrape a thin pass over the whole surface with a wide flex knife, let it dry, sand flush, and dust again. The wall should now feel uniform to a bare hand, run your palm over it; your hand will find defects your eyes miss.
A professional rhythm looks like this: skim on day one (and two, if double-passed), sand and dust the following morning, putty the same afternoon, fine-sand and dust the next morning, prime by midday, and first topcoat the following day. Compressing that sequence into a single day is how rooms end up repainted within the year.
Step 6: Paint
With a primed, puttied, dust-free surface, the painting itself is the easy part:
- Cut in first. Brush the edges, corners, and around fittings before rolling the fields.
- Roll in a W. Load the roller, work a "W" pattern over about a square metre, then fill it in without reloading. This spreads the paint evenly and avoids lap marks.
- Keep a wet edge. Always roll into the previous section while it is still wet. Dry laps show as glossy bars in raking light.
- Respect recoat times. Two thin coats with full drying between them look and last better than one thick coat. Follow the manufacturer's recoat interval, in humid conditions, the safe assumption is the upper end of the stated range.
Quality Checks Before You Sign Off
Professionals judge a finished wall with three tools, not with opinions:
The raking light. Hold a bright torch or work lamp flat against the wall in a darkened room. Every ripple, pinhole, and trowel mark casts a shadow. Do this after each major stage, not only at the end, defects are cheap to fix mid-process and expensive to fix after paint.
The straightedge. Re-run the level test from Step 1. The wall should now show no gap larger than about 2 to 3 mm under the rule.
The eye and hand test. Look along the wall from a shallow angle in daylight and run a bare hand over the full surface. Both should report the same thing: uniform, flat, no grit, no snags.
Hold the final inspection in the actual lighting the room will live under, evening bulb light hides less than you think, and daylight from a window at a shallow angle is merciless. If the wall passes at 4 p.m. with sun through the window, it passes.
A Worked Example: One Living Room, Start to Finish
Take a typical living room: 4.8 m × 3.6 m with a 3.0 m ceiling, one door (0.9 × 2.1 m), and two windows (1.2 × 1.2 m each).
Area. Wall perimeter is (4.8 + 3.6) × 2 = 16.8 m; gross wall area is 16.8 × 3.0 = 50.4 m². Openings total 1.89 + 2.88 = 4.77 m², so net wall area is roughly 45.6 m². The ceiling adds 4.8 × 3.6 = 17.3 m². Total coated area ≈ 63 m², plus edge work at reveals and corners.
System. The straightedge finds 3 to 4 mm ripples, the common case, so the room takes the skim route: skim, putty, primer, two topcoats.
Materials (planning figures). Gypsum skim at about 1 kg/m²/mm for a 3 mm build: ~140 kg, about four 40 kg bags, with one spare for repairs. Putty: one 20 kg tub covers this room's pinholes comfortably. Primer at ~11 m²/L: about 6 L. Topcoat at ~12 m²/L per coat, two coats: about 11 L, three 4-litre gallons or the nearest drum size, with ceiling white separate if ceilings differ.
Labour. At a reasonable pace, a skilled finisher does the skim in a day, sanding and putty in a day, prime and first coat in a day, second coat in a morning, call it four working days for one room at craft pace, plus your own inspection passes with the torch between stages.
The check that pays. The raking-light pass after sanding finds a ridge behind the door, a five-minute fix while the wall is bare. Found after the second topcoat, the same ridge costs a localized re-skim, a feathered sand, a spot-prime, and a full-coat blend across the whole wall plane so the repair does not show. The torch is the cheapest tool on the site.
Timing the Work Around the Seasons
The calendar matters more than the brand name:
Dry season. The friendliest coating window, except in its dustiest weeks, when fine dust is everywhere. Dust is the enemy of fresh coats: cover buckets, work room-by-room, damp-wipe surfaces immediately before coating, and do not sand in the same room where a coat is drying. Very hot afternoon surfaces can skin dry before the coat bonds; morning work on shaded walls avoids most of it.
Rainy season (April–October). Humidity stretches every drying interval, plan recoats at the top of the manufacturer's range, sometimes longer. Never coat a wall that got wet or is respiring after rain; the overnight polythene test is the referee. Ventilation becomes the bottleneck: windows open between showers, fans where power allows.
The rule under both: walls must be dry, the air must be dry enough, and the next four hours must be rain-free (for anything exterior-adjacent) and dust-free (for anything interior). Everything else in this guide survives a bad season; the coats themselves do not.
Who Does the Work: Artisan, Finishers, and Contractors
- The local artisan (per-day or per-job). Affordable, available, and highly variable. Works best when you supply the sequence yourself, the assessment, the system, the stage inspections, because a good hand with no inspection discipline will skip putty and primer as "not necessary" and the wall will agree until the first humid season.
- The specialist finisher / painter. Skim and putty are separate skills from painting; a painter who cannot skim will quote "skim" and deliver a single thick fill pass. Ask for a wall they finished six months or a year ago, not the fresh one.
- The contractor (per m² or job-and-finish). Pricier per metre, but the per-metre rate only means something when the scope is written: skim thickness, sanding, putty, primer brand, coats, and who fixes hollows found mid-job. The cost estimation guide breaks the quote-comparison method line by line.
Whichever route, keep the stage inspections in your own hands: the straightedge pass before coating, the torch pass after sanding, the touch test before priming. They cost minutes and they are the whole difference.
What It Costs: and Where the Money Goes
A realistic interior finishing budget is driven by wall condition more than by paint brand. As a planning frame: on sound plaster, putty and paint with primer typically cost a modest amount per square metre in materials; a wall needing full skim roughly doubles the coating budget and adds materially to labour; walls needing re-plastering belong to a different budget conversation entirely. Material prices move, sometimes weekly, so run current figures through the finishing estimator rather than trusting any static article price list, including this one.
The pattern that holds across every market: labour for assessment, skim, and sanding is where corners get cut, because defects at those stages hide so well under fresh paint, for about six months. Pay for the invisible stages; the visible ones are cheap.
After the Finish: Maintenance and the Repaint Cycle
A finished interior wall is a maintained surface, not a permanent one:
- Wash, do not scrub. Satin and sheen emulsions take a soft sponge and mild soap; matte films scuff if scrubbed. Clean marks early, fresh marks leave, old marks stay.
- Watch the water routes. The first bubbling near a bathroom wall, window, or roof edge is a moisture problem arriving. Fix the water, then the paint; repainting over it is decoration on a timer.
- Touch up from the same batch. Keep a labelled tin of the last topcoat. Local touch-ups from a new batch read as patches in raking light even when the colour code matches.
- Repaint before failure, not after. A repaint cycle of roughly four to six years keeps the film protecting the skim underneath; walls repainted only after peeling begins need repair work, not just paint.
Quick Answers to the Questions Owners Ask Most
Can I paint straight over new plaster? Only after it has cured and dried, roughly 28 days minimum, confirmed with the overnight polythene test. Before that, the paint seals construction moisture in, and the wall will push it back out as bubbles within months.
Is gypsum skim better than cement skim? Neither is better; they are different tools. Gypsum gives a finer, whiter, faster finish for ordinary interior walls. Cement skim costs less per square metre and tolerates damp-prone walls better. Gypsum has no place on exterior or wet-area walls at all.
How many paint coats do I actually need? Over a properly primed surface, two coats is the professional standard. One thick coat is not equivalent, it hides less, fails sooner, and cannot be touched up invisibly.
Do I need to skim if the plaster feels smooth to the hand? Feel is not the test, the straightedge and the raking torch are. A wall can feel smooth and still ripple visibly under paint. If the rule shows gaps under 2 mm and the light is quiet, putty and paint are enough.
Why does my painter want to skip primer? Because primer is invisible in the finished result and its absence shows up after they have left. It is the most skipped coat and the most consequential one. Insist on it, in writing.
How long before I can hang pictures or lean furniture? Emulsion is dry to touch in hours, but full film hardness builds over one to two weeks. Give it a few days before abrading the surface with furniture, and avoid washing the film in the first month.
Conclusion
Interior wall finishing rewards sequence discipline over material spending: assess honestly, choose the system the wall needs, prime what must be primed, skim thin, sand and dust properly, and check the wall with raking light before it gets the chance to check you. For quantities and current costs, use the finishing estimator and the paint calculator; for a deeper comparison of the coating materials themselves, see types of wall finishes: skim coat, putty, and paint.