What is a brush-off blast clean? And when will I ever need to use it?

A brush-off abrasive blast clean, also known as a sweep blast, is an important process for galvanizers and fabricators to understand when working with duplex coatings. Put simply, its an abrasive blasting method gentle enough to keep a galvanized coating intact, while aggressive enough to clean and profile the surface for a second protective coat of paint or powder to be applied. It is described as an acceptable surface preparation procedure for paint in ASTM D6386 Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting, and for powder coating in ASTM D7803 Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Powder Coating.

Procedure

The brush-off blast procedure for paint preparation is identical to the procedure for powder coat preparation as defined in ASTM D6386 and D7803. The stated purpose of the procedure is to deform, not remove the galvanized metal. In other words, a less aggressive procedure than a traditional blast clean should be used to prevent excessive removal of the zinc coating. A profile of at least 0.75 mils should be created while not removing more than 1 mil of the galvanized coating. Below are guidelines operators should follow to avoid removing excessive amounts of zinc while still creating an acceptable profile.

• Use rapid nozzle movement
• Maintain an overall work-rate of 1200 ft²/hr or greater 
• Choose an appropriate blasting medium
  • Aluminum/magnesium silicate with particle size of 8 to 20 mils
  • Soft sands with a MOHs hardness of five or less
  • Organic media, such as ground corn cobs or walnut shells
  • Corundum or limestone
• Select blast pressure based on abrasive medium, nozzle to work distance, and geometry of the piece
  • Even 20 to 40 psi could be too much in some cases
• Use clean, un-contaminated compressed air
• Maintain temperature of the substrate at 5°F or higher than the dew point
• After blasting, blow down surface with clean compressed air
• Apply paint or powder coating as soon as possible after blasting

SSPC-SP 16

Brush-off or sweep blasting of galvanized steel is not only covered by the usual galvanizing standards. Most cleaning practices used by the industry are also covered by SSPC specifications and then referenced by the ASTM standards.

SSPC-SP 16, Brush-Off Blast Cleaning of Coated and Uncoated Galvanized Steel, Stainless Steels, and Non-Ferrous Metals, is a surface preparation standard from the Society for Protective Coatings covering preparation of galvanized steel prior to the application of paint or powder coat. A brush-off blast cleaned surface is defined as free of all visible oil, grease dirt, dust, metal oxides (corrosion products), and other foreign matter. The surface should have a minimum profile of 0.75 mils over the entire surface.

It may be obvious SSPC-SP 16 contains the same fundamental requirements of, and is comparable to SSPC-SP 7, Brush-Off Blast Cleaning, which covers a brush-off blast for carbon steel substrates. These two standards are intended to contain similar cleanliness standards. However, non-ferrous metals such as zinc require different considerations in the blasting process and another standard is needed.