What is Swarf and what to do about it

Swarf is the steel debris created by using friction saws, abrasive discs, drills and other metal cutting machines on steel roofing and wall products. The heat generated by the cutting or drilling tool causes the tiny metal fragments,swarf, to stick to the surface of the panel. If left on the surface of coated steel, swarf will corrode the steel and cause rust stains. It is particularly noticeable when the builder predrills the panels and neglects to wipe the surface clean.

Swarf stains on prepainted steel usually do not affect the performance of the finish. Most of the time, the finish merely absorbs the swarf stain, so the problem is one of aesthetics.

On metallic coatings, however, the zinc in the coating surface will sacrifice itself to prevent the swarf, and the steel underneath, from rusting. Over time, spots will appear in the areas where the zinc has corroded. In damp or humid climates it will happen quickly.

When swarf appears, it’s time to make repairs.

Metallic-coated steel sheet:
Brush the surface with a non-metallic, stiff-bristle brush to dislodge particles.
Do not use steel wool; it breaks down and becomes swarf itself. Be sure to remove the swarf completely. Don’t just sweep it into the building gutters.
Wire brushing will mar the appearance of the sheet, so paint the area immediately.
Painting is especially important if the coating is severely damaged.

Prepainted steel sheet with minor staining:
Most mild swarf stains can be removed using a household cream cleanser. Be sure to follow the cleanser directions.

Prepainted steel sheet with more severe staining:
Clean the surface by washing with a nonionic industrial detergent or household detergent and water. Be sure to use recommended proportions and rinse well with clean water.
Use a stiff nylon brush to remove the corrosion, being careful not to damage the paint film. More heavily affected areas might need a light rub with a Scotchguard-type pad. Do not use steel wool.
Treat affected areas with a rag soaked in a phosphoric-acid-based solution per the supplier’s recommendations. Hose down completely. Normally, this treatment leaves only very mild stains.

Prepainted steel sheet with extensive staining:
When appearance is important, painting over the stains is probably the quickest and easiest solution. Remove any swarf particles left on the surface. If left on the surface, the rust will bleed through the new paint as the particles oxidize.
Repaint the entire visible area. Air-drying paints weather differently and more rapidly than pre-paint systems, so make sure the visible finish is consistent.