Surface preparation and painting

Why surface preparation decides coating life

Most premature coating failures on steel buildings trace back to surface preparation, not to the paint itself. Hot-rolled plate carries mill scale, rust and contaminants; welds leave spatter and flux residue; cut edges are sharp. Removing these and creating a clean, roughened surface is what allows a primer to bond and the system to reach its design life. The paint system is specified separately – see paint and coating systems – but it only performs if the steel underneath is properly prepared.

Shop blasting

Primary PEB members are cleaned in automatic wheel-blast (shot blasting) machines: turbines throw steel shot or grit at the member as it passes through on a roller conveyor. Before blasting, welds are dressed, spatter removed and sharp edges rounded. The standard grade for paint systems on structural steel is Sa 2½ ("very thorough blast cleaning") to ISO 8501-1, equivalent to SSPC-SP 10 / NACE No. 2 near-white blast in North America. Lower grades such as Sa 2 may be accepted for mild internal environments where the specification allows; Sa 3 is reserved for special coatings such as thermal spray.

• Cleanliness – visual comparison with ISO 8501-1 photographs.
• Surface profile – typically medium grade, around 40–75 µm Rz depending on the primer; measured by comparator (ISO 8503) or replica tape.
• Dust and soluble salts – dust to ISO 8502-3, chlorides to ISO 8502-6/-9 where the specification calls for it.
• Climate – steel temperature at least 3 °C above dew point and relative humidity within the paint maker's limits.

Paint application

Paint is applied by airless spray in a booth as soon as possible after blasting to avoid flash rust. Edges, welds and bolt holes are stripe-coated by brush so that sharp corners receive full film thickness. A typical PEB shop system ranges from a single coat of alkyd or epoxy primer for enclosed, dry interiors to a three-coat zinc-rich epoxy / epoxy MIO / polyurethane system for coastal or industrial atmospheres.

Faying surfaces of slip-critical connections are masked or coated only with a primer qualified for the required slip coefficient (RCSC Specification in the US, EN 1090-2 Annex G in Europe). Areas to be welded on site and anchor zones embedded in concrete are left uncoated as detailed.

Hot-dip galvanizing as an alternative

For agricultural buildings, coastal sites and exposed canopies, hot-dip galvanizing to ISO 1461, ASTM A123 or AS/NZS 4680 gives a metallurgically bonded zinc coating that protects edges and inside faces equally. Members must be detailed for galvanizing (vent and drain holes, limits on bath size) and large tapered members may need to be split. A duplex system of galvanizing plus paint offers the longest life in C5 conditions. See agricultural buildings for a typical use.

Touch-up, repairs and painting galvanized steel

Coating damage during handling, transport and erection is normal; what matters is that it is repaired with a compatible system. Typical rules:

Painting over galvanizing. New zinc surfaces are smooth and reactive, so paint needs a prepared surface: light sweep blasting with non-metallic abrasive or chemical pretreatment, followed by a primer suited to zinc (for example an epoxy tie coat). Guidance is given in ASTM D6386 and ISO 12944-4. Alkyd paints applied directly to zinc are prone to peeling and should be avoided.

Inspection and records

Wet film thickness is checked during spraying and dry film thickness (DFT) is measured with magnetic or eddy-current gauges after curing, using the sampling and acceptance rules of ISO 19840 or SSPC-PA 2. Adhesion may be tested by pull-off (ISO 4624) or cross-cut (ISO 2409) on test panels. Results are recorded in the coating report included in the shipment dossier – see quality control and inspection. World Steel blasts primary members to Sa 2½ on automatic lines and applies paint systems from suppliers such as Jotun, International Paint and Hempel as specified for each project.