The integrity of structural steel depends in part on how effectively it can be protected from rust. The standard way to achieve this is to cover the steel in a protective coating. However, it is crucial that the coating is applied on steel that is clean, otherwise corrosion may occur below the surface of the coating – creating bubbles over weakened parts of the metal.

As steel comes freshly rolled from the mill, it is already covered in mill scale, a thin layer of iron oxide. As soon as the mill scale layer is broken by handling, impact or friction, it allows corrosion of the base metal. To make matters worse, any paint applied over mill scale will be ineffective, as the corrosion will take place below the paint surface, damaging the metal itself. So it is imperative that mill scale is removed from structural steel before it is coated – for instance, by abrasive blasting.

The surface to be covered should be free of not only mill scale, but any dirt, grease or oil, and it should be perfectly dry. The aim is to remove anything that may hamper the adhesion of the coating to the metal and to create some roughness on the surface to improve adhesion. A protective coating works only as long as it sticks to the metal surface and its own surface remains unbroken.

Four grades of cleanliness are generally specified:

  • White metal blast-cleaned – No visible mill scale, rust or dirt.
  • Near white metal blast-cleaned – A small, set percentage of visible impurities allowed.
  • Commercial blast-cleaned – Nearly all mill scale, rust and dirt removed, but some variation in appearance allowed. This is usually good enough for most uses of structural steel.
  • Brush-off blast-cleaned – No loose mill scale, rust or dirt. This is still better than what usually achieved with power tools or hand sanding.

Other than simply looking at the metal surface, the easiest way to test how clean it is is to wipe it with a clean, white cloth, pressing down firmly. Then check the material for residue, whether rust, mill scale or dirt.

Once the surface is cleaned, it is recommended that you get it covered in a protective coating within four hours or less, otherwise corrosion may start again.

For a gentler clean on your equipment try our Sodablasting Service Sodablasting is safe, environmentally friendly and
  • Cleans steel, metal, fibreglass and concrete surfaces
  • Strips paint from vehicles, boat hulls & masonry
  • Effective and non toxic on food processing equipment
  • Tough on graffiti & contaminates in the industrial sector
  • Is a non destructive cleaning and stripping process


"Tauranga Sandblasting's team constantly delivers high quality work carried out Safely and on time for the last ten years."

Gull New Zealand Limited

"For the last 7 years we have used Tauranga Sandblasting to blast and re-paint onsite structural steel, refrigeration pipe work and ancillary equipment. Where it is possible, any equipment is relocated to their enclosed and environmentally protected processing facility.

Tauranga Sandblasting have always carried out any work in a professional manner, with due concerns for any environmental, operational and compliance issues before they arise. In conjunction with a paint supplier's specification, we have utilised their experience in recommending  and applying paint systems for long term applications in our harsh environment.

To date, all work done is performing to specification which reflects Tauranga Sandblasting's attention to suitability, preparation and application of paint systems."

Peter B. Martin - Sanford Limited, Tauranga