Difference between 'Class of Blast' and 'Surface Profile'

Surfaces must not only be cleaned to the correct class of blast but also have a uniform surface profile

The difference between class of blast and surface profile has been a confusing point for many blasters and painters over the years. For a good industrial coating system to be successful, the steel surface must not only be cleaned to the correct 'class of blast' but the entire surface must be uniformly 'etched or roughened' to provide a surface profile or anchor pattern for the coating to lock onto as it dries.

Class of Blast

Class of blast is the degree of removal of rust, mill scale and other visual contaminants from the steel substrate when inspected without magnification. All blasting preparation grades must be free from all oil, grease and dirt. One of the most frequently used blast of blast standards is class 2-1/2 for a near-white blast.

This specification requires that near-white metal blast cleaned surface, when viewed without magnification, shall be free of all visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products and other foreign matter, except for staining as noted. Random staining shall be limited to no more than 5 percent of each unit area of surface as defined, and may consist of light shadows, slight streaks, or minor discolorations caused by stains of rust, stains of mill scale, or stains of previously applied coating.

Surface Profile

Surface profile is concerned with the 'shape of the surface' finish. When any surface (and particularly a steel surface) is blasted, the abrasive particles 'dig' into the surface on impact and rebound outwards. This impact creates an indentation into the surface a few thousandths of an inch deep, and on rebounding brings some of the surface out and above the surface. This indentation is called a 'valley' and the rebounding creates a 'peak'.

blast surface smooth
Figure 1: Un-blasted "Smooth" Surface
surface blast
Figure 2: Blasting Surface
blast surface profile
Figure 3: Blasted "Profile" Surface

Surface profile is measured in mils, as coating thickness is measured. The surface profile is measured from the bottom of the lowest valley to the top of the highest peak.

Surface profile is critical to coating performance by:

  1. Increasing the surface area
  2. Providing a ‘tooth pattern’ for the coating to lock and adhere to

blast class chart