Cleaning metal cladding and sandwich panels: what to watch out for

Production halls, warehouses and many office buildings have a metal facade that stays crisp for years, until dust, soot and green growth settle where the rain doesn't reach. Below you'll find the types of cladding, why cleaning matters for the coating and what to watch out for. For an overview of all methods, see facade cleaning methods.

What types of metal cladding are there?

Mainly sandwich panels, profiled steel sheet and aluminium cladding. Their finish determines how you clean them.

A sandwich panel consists of two metal sheets with insulation in between. The outer sheet is usually steel that has been painted on the coil in the factory, known as coil coating or pre-finished steel. Profiled sheet is the same kind of painted steel without an insulating core. Aluminium cladding, such as cassettes and panels, is usually powder-coated or anodised. Anodising thickens the natural oxide layer on the aluminium by an electrochemical process (Alumeco).

Why clean sandwich panels and metal cladding?

Because dirt that stays put can harm the coating, and because some manufacturers link cleaning to their warranty.

According to ArcelorMittal, rainwater usually keeps exterior surfaces clean enough, but dirt the rain doesn't wash away should be removed regularly to extend the life of organic coated steel. This prevents poultice corrosion, which forms under a layer of dirt that holds moisture (ArcelorMittal). Kingspan writes that dirt in areas not washed by rain, such as under overhangs, spoils the appearance and could cause the coating to break down, and that airborne impurities weigh heaviest near polluting industry and on the coast (Kingspan).

ArcelorMittal makes respecting its cleaning constraints part of the conditions of its Granite Automatic Guarantee. Kingspan calls records of inspection and maintenance an integral part of its warranties. Tata Steel requires no inspection or maintenance for its Confidex Guarantee on Colorcoat HPS200 Ultra and Colorcoat Prisma, except on roofs and walls with a solar installation (Tata Steel). For powder-coated aluminium, QUALICOAT UK & Ireland says the coater provides a minimum cleaning regime that must be followed and recorded (QUALICOAT UK & Ireland). So read the terms for your own building and keep a record of every clean.

What to watch out for when cleaning metal cladding?

No abrasives, a product that suits the coating, a pressure the manufacturer allows, and a thorough rinse. Cleaning is also the moment to look at joints, fixings and cut edges.

No abrasives or solvents. ArcelorMittal and Kingspan both advise avoiding organic solvents and abrasive cleaners on coated surfaces, and warn that over-cleaning or excessive scrubbing does more harm than good. Both describe a hose, a soft-bristle brush and fresh water, with a mild detergent for heavy industrial deposits.

pH and compatibility. For organic coated steel, ArcelorMittal prefers a neutral pH, because acidic and highly alkaline products can damage coil coating paints. For anodised aluminium, Alumeco writes that a pH below 5 or above 8 dissolves aluminium oxide and causes lasting damage. If in doubt, ask the panel manufacturer which product is allowed.

Pressure, temperature and contact time. For its own organic coated steel, ArcelorMittal asks for a rinsing pressure below 5 MPa, which is 50 bar, generally at a temperature below 30 °C, and leaves a cleaning product on the coating for no more than 30 minutes. Alumeco notes that a high-pressure cleaner is common on anodised aluminium, provided the aluminium can withstand the pressure. That limit applies to ArcelorMittal's own steel. Ask your panel supplier for its cleaning instructions.

Rinse, and test first. ArcelorMittal advises cleaning from top to bottom, rinsing immediately and thoroughly with clean water, and testing a product on a small area first.

Joints, fixings and cut edges. Faulty or inappropriate fasteners can cause leaks or rust staining, and drilling swarf left from installation can rust and stain (Kingspan). According to ArcelorMittal and Kingspan, corrosion at cut edges on sheet overlaps and overhangs can spread up the sheet if ignored. Don't aim the jet straight into joints and seals either, and report what you see.

Rooflights and working at height. The UK Health and Safety Executive lists rooflights in the roof plane among the materials likely to be fragile, and notes that people doing small, short maintenance and cleaning jobs are over-represented in the injury statistics. Its first step is to plan the work so that people stay away from fragile surfaces (HSE).

What soiling builds up on industrial facades?

Mainly industrial dust and soot, green growth on the damp sides and bird droppings under roof edges and on ledges.

Green growth and mould grow where it stays damp, for instance near air-conditioning units or under trees, according to Kingspan. Why north-facing walls turn green faster and how to deal with it is covered in green algae on facades. Build-ups of bird droppings or leaves are best removed quickly: ArcelorMittal asks for accumulated debris to be removed promptly to prevent poultice corrosion.

How often should cladding panels be cleaned?

Follow the manufacturer of your panels. For coated steel, the sources here assume an annual inspection and washing sheltered areas at least twice a year.

BlueScope advises hosing down parts that rain doesn't wash, such as wall cladding under eaves, at least every six months, and more often on the coast and where industrial fallout is high (BlueScope). Kingspan recommends an annual inspection and washing walls ideally at least every six months, every three months on the coast. For aluminium, QUALICOAT UK & Ireland (powder-coated, in an urban setting) and Alumeco (anodised) suggest roughly one wash a year.

Frequently asked questions