Painting Interior Walls

Read the wall before you price the work

Every figure on a paint takeoff traces back to one question, and it is not how many square metres the room measures. It is what the wall will do to the first coat that lands on it. Porosity decides how much of that coat soaks away instead of forming film. Adhesion decides whether it stays there. Colour and contamination underneath decide how many coats are needed to bury them. Two rooms of identical dimensions can differ by half again in material and by a full day in labour, and the whole difference is legible on the surface before anyone cracks a lid.

Walk the room with four checks in your pocket. Wet a fingertip and touch the wall: water that vanishes on contact means high suction, so bare board, fresh compound, or a chalky old distemper. Water that beads means gloss, silicone residue, or a stain-release additive still doing its job. Rub an inconspicuous patch with a rag damped in denatured alcohol; a film that softens and comes away on the cloth is a water-sensitive distemper or calcimine that nothing modern will grip. Cut a lattice through the existing coating and lift it with tape per ASTM D3359 Standard Test Methods for Rating Adhesion by Tape Test, and the decision to overcoat or strip stops being an argument. Put a moisture meter on anything below grade, beneath a bathroom, or on an external elevation.

Record the answers wall by wall rather than room by room. One elevation of unpainted patched board in an otherwise sound bedroom drives a spot-prime line, not a full prime line, and pricing it as a full prime is how a competitive quote stops being competitive.

New board and fresh joint compound

Fresh board is two substrates pretending to be one. The paper face has a mild and reasonably even absorbency; the joint compound over tapes, screw heads and butt joints drinks far harder. Roll a topcoat straight onto that assembly and the compound strips binder out of the wet film, leaving dull, slightly darker bands exactly where the finisher worked. Joint banding shows from the doorway under any raking light and cannot be cured by adding topcoats, because each new coat meets the same suction differential. A dedicated drywall primer-sealer earns its place here by equalising suction, not by hiding colour.

Specify the finish level before the coating, because the two are bound together. ASTM C840 Standard Specification for Application and Finishing of Gypsum Board and the Gypsum Association's GA-214 Recommended Levels of Finish for Gypsum Board, Glass Mat and Fiber-Reinforced Gypsum Panels set out the levels a painter inherits from the boarder. A wall finished to a lower level, then given an eggshell or gloss under wall-washer lighting or a large window, will telegraph every joint and fastener. No primer rescues that.

Sanding dust is the other quiet failure on new work. Compound dust left in the joint feathers and along the base acts as a release layer, and the first sign is a coat that peels in sheets when tape is later applied for a second colour. Vacuum with a brush head, then wipe with a barely damp sponge and let the board dry before priming, and never wet-sand into a joint you intend to prime the same day.

Sound previously painted latex

Sound previously painted latex is the cheapest wall in the house to paint, and the estimating trap is assuming every wall in the house is that wall. Coverage runs at or near the manufacturer's stated spread rate because the existing film is already a sealed, uniform substrate. Washing still matters: kitchens carry an aerosol grease film, hallways carry hand oils at door height, and both defeat adhesion in exactly the places that get handled most. Warm water and a proprietary degreaser, rinsed, is the whole preparation on a genuinely sound wall.

Colour change is what actually drives coat count on this substrate, not condition. Going darker over light, or covering a saturated red, yellow or orange, is a hiding problem that a tinted primer solves more cheaply than a third finish coat, because primer is formulated to be opaque and cheap to apply rather than to be scrubbable. Sheen change deserves the same thought in the opposite direction: dropping from a satin to a matt hides substrate flaws, while climbing from matt to satin exposes every one of them and quietly converts a two-coat job into a fill-and-sand job.

Patches within an otherwise sound wall behave like fresh compound, because that is what they are. Spot-priming them is not optional tidiness; an unprimed patch flashes through two finish coats and shows as a dull halo under sidelight. Feather the compound wide, prime the patch and a hand's width beyond, and let the primer flash off fully before the finish coat goes on.

Gloss, enamel and oil-bound films

Adhesion, not coverage, governs every glossy wall. Alkyd enamels, older oil-bound systems, varnished panelling and factory-finished trim all present a closed, low-energy surface that a water-borne topcoat will sit on rather than bond into. The failure is delayed and expensive: the film looks perfect for months, then lifts in a sheet the first time a picture hook comes out or a strip of masking tape is pulled during the next redecoration.

Abrade first, then bond. Mechanical keying with 120 to 180 grit gives the surface profile that chemistry alone will not; liquid deglossers soften and dull a film but leave the profile shallow, so treat them as a supplement on mouldings and detail where sanding cannot reach, not as a substitute on flat runs. Follow with a bonding primer specified for glossy substrates, and confirm the result rather than assuming it, with a tape test on a sample area once the primer has cured to its stated recoat time.

Saponification is the specific failure that catches alkyd on an alkaline wall. Where an oil-based coating has been applied over fresh plaster, lime or concrete that has not carbonated, alkali and moisture attack the binder and the film turns soft, sticky or soapy, sometimes years later. The wall must dry and the coating must be stripped back to sound material; overcoating a saponified film simply buries a soft layer under a hard one.

Plaster: lime, veneer and lath

Lime plaster and traditional lath work sit at the opposite end of the porosity range from gloss. Green plaster is both wet and strongly alkaline, and it needs to carbonate before it takes a low-permeability coating. The drying period varies enormously with thickness, background, ventilation and season, so it is governed by site conditions and the plasterer's specification rather than by a number that travels between jobs.

Alkali-resistant primer is the correct answer on plaster that must be painted before it is fully mature, and a breathable mineral or contract emulsion is the correct answer on lime work that needs to keep passing moisture. Gypsum veneer plaster behaves differently again: it is dense, low in suction and smooth, so the problem is grip rather than absorbency, and a plaster-rated primer beats the drywall sealer that would be right one substrate over.

Hollow areas need resolving before any coating decision. Tap the wall with a knuckle across each elevation; a drummy note over lath means the keys have failed behind the plaster and the section will move again regardless of what is painted onto it. Reattachment or replacement is a plastering line, not a painting line, and pretending otherwise puts a crack through new work inside a season.

Contaminated walls: nicotine, soot, grease and staining

Tar and dye contamination is a bleed-through problem, and water-borne coatings make it worse before they make it better. Nicotine, soot from a chimney or candle burning, and the residue from a kitchen fire all contain water-soluble and solvent-soluble colourants that a wet emulsion redissolves and lifts into the fresh film. The result is a brown or yellow bloom that grows more obvious as the coat dries, and a second coat repeats the performance.

Wash before blocking, because a stain blocker applied over a greasy film simply seals the grease in place under a coating with nothing to bond to. Degrease with a proprietary cleaner, rinse, allow full drying, then apply a shellac-based or solvent-borne stain-blocking primer that carries no water to reactivate what is left. Ventilate properly and check the product's flash point and respiratory guidance, because these are the highest-solvent materials on an interior job.

Mould is not a coating problem at all. Visible growth means a moisture source, and a fungicidal wash followed by paint hides the symptom for one season while the cause continues. ANSI/IICRC S520 Standard for Professional Mold Remediation sets out the remediation approach where that standard is adopted, and the practical sequence on site is the same everywhere: find and fix the water, dry the fabric to a stable reading, remove contaminated material, and only then decorate.

Concrete, block and masonry on the inside face

Concrete, blockwork and dense masonry on an interior face fail an estimate on texture rather than on chemistry. The stated spread rate on a tin assumes a smooth substrate; open-textured block can consume several times that on the first coat purely in filling voids, and rolling more topcoat into an open surface is the most expensive way to fill it. A block filler applied first levels the profile and leaves a substrate the finish coat can cover at something near its rated rate.

Two properties then decide the primer. Alkalinity from cement demands an alkali-resistant system for the same reason green plaster does, and residual moisture demands patience. ASTM D4263 Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method gives a crude but useful field indication, while ASTM D4258 Standard Practice for Surface Cleaning Concrete for Coating and ASTM D4259 Standard Practice for Abrading Concrete cover the preparation those substrates need.

White crystalline bloom on the surface is efflorescence, and it means salts are being carried out by moving moisture. Brushing it off and painting traps the next generation of salts behind the film, which then blisters. Deal with the water path first; the coating decision follows.

Stripped wallpaper and skim-coated repairs

Wallpaper that has been stripped leaves an adhesive residue that no amount of visual inspection reliably finds. Old paste is hygroscopic and rewets under a water-borne coating, producing bubbles, wrinkles and a surface that lifts under a fingernail a day later. Wash the wall down thoroughly with hot water and a paste remover, rinse, and let it dry fully; where the residue is heavy or the substrate is unlined board, an oil-based or shellac-based sealer is the safe route because it introduces no water to reactivate what remains.

Torn face paper is the other legacy of a stripped wall, and it must be sealed before filling. Exposed gypsum core and shredded paper both swell when compound is applied over them, lifting the repair and blistering the paper edges. An oil or pigmented shellac sealer over the torn area stabilises it, then fill, sand, spot-prime and proceed as with any patch.

Skimming a whole wall resets the substrate to bare compound and resets the estimate with it. Everything covered above about fresh joint compound applies to a skim coat across its entire area, so the primer line goes from spot to full and the drying time goes into the programme.

Conditions, sequence and the cost of a wrong call

Coatings behave to their data sheet only within the temperature and humidity window that data sheet states. Cold slows cure and can leave a film that never develops its stated hardness or block resistance; high humidity extends recoat times far beyond the printed figure and can leave a matt finish patchy in sheen. Substrate temperature matters more than air temperature on external walls and unheated rooms, and painting a cold wall in a warm room is a reliable way to produce condensation under a fresh film.

Sequence protects the work. Ceilings before walls, walls before trim, and cutting-in kept wet-edge to the roller so laps do not flash on a matt finish. Where a full skim or a heavy stain-block is in play, build the extra drying into the programme before the trades that follow are booked in.

Regulation sits over all of it and varies by jurisdiction. Disturbing paint in older housing brings lead into scope: in the United States the EPA Renovation, Repair and Painting Rule applies to pre-1978 target housing and child-occupied facilities, while in Great Britain the Control of Lead at Work Regulations govern; other jurisdictions have their own instrument, and the local rule is what binds. Volatile organic compound limits are similarly local, set by the EPA architectural coatings rule and by stricter regional authorities in parts of the United States, and by Directive 2004/42/EC in the European Union, which is why a stain-blocking primer available on one job may not be available on another. Confirm both before specifying a solvent-borne product, not after it arrives on site.

1. Survey each elevation and log its substrate condition separately.
2. Test any glossy or unidentified film for adhesion before deciding to overcoat.
3. Meter below-grade, bathroom-adjacent and external walls for moisture.
4. Resolve water sources, mould and hollow plaster as separate trades.
5. Clean, then block stains, then prime to the substrate's governing property.
6. Apply finish coats to the hiding requirement the colour change sets.