External Coating Systems

External coating systems are the paints, renders, texture coatings and protective coatings on the outside of a building. Some are decorative, but most do a job: keeping water out of masonry and render, stopping carbonation reaching reinforcement in concrete, protecting steel from corrosion and timber from decay. When a protective coating fails, the element beneath starts to fail.

Older buildings carry many layers. In Australia, paint applied before about 1970 commonly contains lead; in the United States, the cut-off for housing is 1978; in Britain, lead paint was common until the 1960s and in some uses later. Thick stacks of old paint crack and peel as a unit, and their removal is a hazardous materials operation.

What shortens life:

• Preparation: coating applied over dust, chalk, mould, loose old paint or damp substrates fails early, whatever the product.
• Exposure: UV on north and west elevations in Australia, wind-driven rain, coastal salt, industrial atmospheres.
• Moisture behind the coating: rising damp, leaking gutters, failed flashings and sills.
• Incompatible layers: acrylic over gloss enamel without preparation; impermeable paint over lime render or soft brick.
• Substrate movement: cracking sheet joints, moving timber, cracking concrete.
• Too many coats: thick stacks crack and peel as a unit.

Chalking and fading

UV breaks down the binder at the surface, leaving loose pigment that wipes off as a powder. It is the normal ageing of exterior paint and the usual cue for a recoat.

Loss of adhesion

Paint peels from the substrate when moisture builds up behind it or it was applied over a poor surface; it peels between coats when a new coat was applied over a glossy, dirty or incompatible older coat.

Blistering

Blisters form when moisture vapour from inside the wall pushes against an impermeable film, or when solvent or water is trapped during application. Blisters full of water point to moisture behind.

Cracking

Brittle old alkyd and oil paints crack in a fine pattern as they age (checking, alligatoring). Coatings crack along lines that follow joints in sheet substrates and cracks in concrete or render below.

Render debonding and cracking

Render loses its bond to masonry when water gets behind it or when it is stronger than the backing; it sounds hollow, bulges and falls. Fine map cracking is drying shrinkage; cracks along regular lines follow the substrate.

Coating failure on concrete

Where an anti-carbonation or protective coating fails, carbonation advances to the reinforcement, which corrodes and spalls the concrete. Rust staining through a coating is the sign that it is already happening.

Breakdown on steel

Coatings on steel break down at edges, welds, bolts and damage points first, then rust spreads under the film. Galvanising under the coating extends life until it too is consumed.

Lead paint deterioration

Deteriorating lead paint chalks, flakes and sheds lead dust and chips onto the ground and into interiors, which is a health hazard, particularly to children.

From the street and from aerial imagery

• Faded and patchy colour; mismatched patches from local repairs.
• Peeling and flaking, worst at sills, the base of walls, under gutters and on the most exposed elevations.
• Cracked and bulging render; missing render patches exposing the masonry.
• Rust staining through coatings on concrete; rust at edges and fixings of coated steel.
• Mould, algae and dark staining on shaded elevations.

Close up on site

• Chalk on a finger wiped across the surface.
• Peeling to bare substrate or between coats; thickness of the paint stack at a peeled edge.
• Blisters: dry or water-filled.
• Hollow sound of render when tapped; cracks following sheet joints.
• Coating cracks tracking cracks in the concrete beneath; spalling under coatings.

Inside the building

• Damp at the inside face of walls with failed coatings, and blistering of internal paint on external walls.

In the records

• Repaint history with dates, products and preparation specifications.
• Hazardous materials register and lead testing.
• Coating warranties and concrete repair records.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

A coatings inspector or building surveyor leads; a concrete repair specialist joins where reinforcement corrosion is suspected.

• Condition rating of chalking, cracking, flaking, blistering and rusting against the ISO 4628 series.
• Adhesion testing by cross-cut (ISO 2409 or ASTM D3359) or pull-off (ISO 4624 or ASTM D4541).
• Film thickness measurement on steel and on concrete coatings.
• Render tapping survey to map hollow areas.
• Carbonation depth testing with phenolphthalein, and cover survey, on coated concrete.
• Lead testing by laboratory analysis of paint chips or by XRF on buildings of the relevant era.
• Moisture investigation where peeling or blistering points to water behind.

The report should state the coating system and its condition, the cause of failures, substrate repairs needed, hazardous content, and a specification for preparation and recoating.

• Wash and recoat where the coating is chalked but sound.
• Spot preparation and recoat where failure is local.
• Full removal and recoat where the stack has failed or is incompatible; under lead controls where lead is present.
• Fix the water first: gutters, sills, flashings and damp sources, or the new coating will fail the same way.
• Render: cut out and replace debonded areas, or strip and re-render, with a mix compatible with the substrate.
• Concrete: repair spalls and treat reinforcement to a recognised concrete repair standard, then apply an anti-carbonation coating.
• Steel: blast or mechanically clean, then apply a protective system specified for the site corrosivity category.
• Heritage masonry: remove impermeable paints and use mineral paints or limewash.

Recoating is priced by area with preparation rated separately; substrate repairs (render, concrete) are priced by area or per repair. Access often dominates on multi-storey buildings. Lead removal adds containment, waste disposal and clearance testing. Recoating recurs on a cycle set by the coating and exposure.

Recoating is lifecycle; patching is repairs and maintenance; lead management is code compliance; concrete repair before recoating is repairs or lifecycle depending on extent.

• Australia. AS/NZS 2311 guides the painting of buildings, AS/NZS 2312 the protection of structural steel, and AS 4361.2 the management of lead paint in residential, public and commercial buildings. On strata buildings, external repainting is common property and a regular sinking-fund item.
• United Kingdom. BS 6150 guides the painting of buildings, BS EN ISO 12944 the protection of steel, BS EN 1504 concrete repair and protection, and BS EN 13914-1 external render. The Control of Lead at Work Regulations 2002 govern work on lead paint.
• United States. The EPA Renovation, Repair and Painting Rule (40 CFR Part 745) requires lead-safe work practices on housing and child-occupied facilities built before 1978; OSHA 29 CFR 1926.62 covers worker exposure. AMPP (formerly SSPC and NACE) standards govern protective coatings on steel.

• Repaint history with dates, products and specifications.
• Coating warranties.
• Hazardous materials register and lead test results.
• Concrete repair and carbonation testing records for coated concrete.
• Records of damp and leak repairs on affected walls.

• Colour and product schedule for the current coating.
• Heritage constraints on colours and materials.