Painting a Building Properly: Preparation, Systems and Film Thickness

Paint fails because of what was underneath it or what was skipped before it. This is the sequence from substrate testing to the final coat, with the checks at each step.

Paint is the thinnest layer on the building and the one everyone judges. It is also almost entirely dependent on what it is applied to: a damp substrate, an alkaline new plaster, a dusty surface or a previous coating that is failing will all defeat an excellent paint. A paint system is a sequence, primer and intermediate and finish coats designed to work together, applied at a stated thickness. Treating it as a colour applied twice is why external walls in the Kingdom need repainting so often.

Before stage one

Nothing should be painted before the substrate has been tested and the system selected to suit it. The decisions that matter are the moisture content, the alkalinity of new cementitious surfaces, the soundness of the surface and whether the exposure is internal, external, wet or subject to constant sun. These determine the primer, which determines everything after it.

• Substrate moisture measured and within the limit the paint system allows.
• Surface soundness checked, with hollow or friable areas repaired rather than painted over.
• A full paint system specified by the manufacturer for this substrate and exposure, with the primer named.
• Colour and sheen approved on a sample panel viewed under the lighting of the finished space.
• Protection and masking arranged for finished floors, joinery, glazing and ironmongery.

Stage 1: Test the substrate and record what you find

Measure moisture content with an appropriate instrument, check alkalinity on new cementitious surfaces, and test adhesion of any existing coating by a simple tape or cross hatch test. Record the readings by area. These numbers decide whether the surface is ready, and they are the only defence if a coating fails and the cause is disputed.

• Deliverable: A substrate test record by area covering moisture, alkalinity and existing coating adhesion.
• Owner: Painting subcontractor, verified by the supervising engineer.
• Approval gate: No preparation starts until the substrate readings are accepted.

Stage 2: Repair the substrate before you prime it

Cut out hollow plaster, fill cracks appropriately for their cause rather than simply skimming them, make good around fixings and penetrations, and remove any contamination such as release agent residue, salts or mould. Paint does not bridge a defect, it reveals it, and a crack filled without addressing movement will reappear through the paint.

• Deliverable: A completed repair record with photographs of each repaired area before priming.
• Owner: Contractor finishing team.
• Approval gate: Repairs inspected and accepted before any primer is applied.

Stage 3: Clean and prepare the surface mechanically

Remove dust, laitance and loose material by the method the system requires, which may be brushing, abrading or washing followed by full drying. On external surfaces, dust is continuous in this region, so the gap between cleaning and priming should be short. A surface that was clean yesterday is not clean today.

• Deliverable: A preparation sign off immediately before priming, with the method recorded.
• Owner: Painting subcontractor.
• Approval gate: Priming follows preparation closely, within the period the specification allows.

Stage 4: Apply the primer the system specifies

The primer does the work: it seals a porous substrate, resists alkalinity on new plaster and concrete, promotes adhesion and stabilises a chalky surface. Substituting a cheaper primer, or diluting the finish coat and calling it a primer, removes the function the system depends on and is the single most common cause of coating failure.

• Deliverable: An application record naming the primer product, batch, dilution and coverage achieved.
• Owner: Painting subcontractor.
• Approval gate: Primer confirmed as the specified product before the intermediate coat.

Stage 5: Apply the coats at the specified thickness and intervals

Each coat has a wet film thickness that delivers the intended dry film thickness, and a minimum and maximum interval before the next. Too thin and the coating does not perform; too thick in one pass and it may not cure properly; outside the overcoating window and adhesion between coats suffers. Verify by comparing the paint consumed against the area covered.

• Deliverable: A coat by coat record with product, thickness and interval, plus litres used against area covered.
• Owner: Painting subcontractor, checked by supervision.
• Approval gate: Each coat accepted before the next is applied.

Stage 6: Control the conditions during application

Paint manufacturers state limits for substrate and air temperature, humidity and wind, and a wall in direct sun in a Saudi summer exceeds them comfortably. Painting a hot surface causes rapid drying, poor flow, visible lap marks and reduced adhesion. The practical answer is to follow the sun around the building and paint shaded elevations.

• Deliverable: A daily record of substrate and air temperature and conditions at the time of application.
• Owner: Site supervisor.
• Approval gate: Application stopped when conditions fall outside the manufacturer limits.

Stage 7: Inspect, measure and hand over the system record

Inspect the finished work for uniformity, coverage, sheen consistency, lap marks and crisp lines at junctions, under the lighting that will exist in use. Where dry film thickness is specified, measure it by the appropriate method. Then compile the record: products, batches, thicknesses, substrate readings and conditions, which is also what any warranty will require.

• Deliverable: A paint system record for the handover file, with the maintenance and recoating guidance.
• Owner: Contractor quality engineer.
• Approval gate: Records accepted as part of practical completion for the painting package.

Handovers between stages

The handover into painting is from the plasterer and the joiner, and the critical transfer is a substrate that is dry, sound and within tolerance, with its moisture reading recorded. The handover out of painting is to the trades doing final fixing, who will inevitably mark the walls, which is why a final touch up visit should be programmed and priced rather than argued about. Keep the same batch of paint on site for that touch up, because a later batch can differ in shade enough to be visible on a wall.

How you know it is finished

Painting is complete when the substrate readings, the system record and the condition log exist, when the finish is uniform under the final lighting rather than under site lights, and when the touch up has been done after the other trades finished. Keep the product and batch information and the recoating guidance; the first time the external walls need attention, the question will be which system is already on them, and that answer determines what can be applied over it.

Domira builds and supervises projects across Saudi Arabia, and this is the level of detail its site teams work at. Send the drawings or the site, and Domira will tell you what the specification has to say before anyone prices it.