Metal Surface Care

Architectural metal on commercial buildings is almost always a coated or finished product rather than bare material, and the finish is what provides the corrosion resistance. Deterioration begins where the finish is breached — at fasteners, cut edges, impact damage, and abrasion — and proceeds from those points rather than uniformly. Preserving metal is therefore largely about preserving the finish.

Common Finishes and What They Tolerate

• Factory-applied coil coatings on panels and cladding are durable and reasonably chemical-tolerant, but can be dulled by abrasive contact and attacked by strong alkaline products.
• Anodised aluminium is resistant but can be etched by acidic and strongly alkaline products; damage appears as patchy discoloration and is not reversible.
• Powder coating is durable and generally tolerant, though ultraviolet exposure produces chalking over time much as with paint.
• Galvanised steel relies on a sacrificial zinc layer; aggressive cleaning removes it and shortens service life substantially.
• Stainless steel resists corrosion but shows water spotting and can develop surface staining from contact with carbon steel particles.
• Weathering steel is designed to develop a stable oxide layer; cleaning it disrupts a protective mechanism rather than removing soil.
• Copper and zinc develop intentional patinas; cleaning may remove a finish the architect specified.

The last two are worth flagging specifically because they are frequently mistaken for neglect. A weathering steel panel or a patinated copper element is performing as designed, and cleaning it is a material change rather than maintenance.

Where Corrosion Starts

Cleaning Without Damage

1. Identify the finish first. Anodised, galvanised, and patinated surfaces each rule out approaches that are fine on coated panel.
2. Use low pressure. Pressure sufficient to clean concrete will dull finishes and can drive water behind panels.
3. Avoid abrasive contact entirely — brushes, pads, and grit carried in runoff all mark finishes permanently.
4. Avoid acidic products on anodised and galvanised surfaces.
5. Rinse thoroughly, since residue on metal can stain and continue reacting.
6. Direct spray downward at laps and joints, not upward, to avoid driving water behind panels.
7. Address rust bleed at its source rather than repeatedly cleaning the stain, which returns as long as the corroding element remains.

Rust Staining Is a Symptom

Rust streaking on an adjacent surface — masonry below a metal element, concrete beneath a railing, cladding under a fastener — indicates something is corroding upstream. The stain can be treated; it recurs until the source is repaired, replaced, or protected.

This is one of the clearest cases where repeated cleaning is a recurring cost with a fixed alternative. Identifying the corroding element during inspection and routing it to repair costs less over a few years than treating the stain indefinitely. See efflorescence, rust, and mineral staining.

What Sits Behind the Panel

Metal cladding systems are frequently rainscreen or drained assemblies, meaning they expect some water behind the panel and are designed to drain it. That design assumption does not extend to water driven in under pressure at a joint or termination.

The practical caution is the same as for other panelised systems: keep pressure low, direct spray downward at joints, and treat sealant and flashing terminations as protected zones rather than surfaces to be cleaned aggressively.