Standards of Surface Finish Chart – The surface under examination is visually compared with high quality photographs in the ISO 8501-1 standard both before and after blasting. This section of the standard is pictorial and show rust grades prior to blasting and the degree of surface cleanliness afterwards. The preparation is then given a coding, e.g. SA2½ which is interpreted using the Standards Wall Chart.
Surface finish from abrasive blasting
This depends upon the initial surface condition as well as the operation of abrasive blasting and the abrasive size, hardness and shape.
Initial surface condition of steel
These can be categorised as:
Rust Grade A
Steel which is completely covered by mill steel.
Rust Grade B
Steel where mill scale has started to flake and light rusting occurs.
Rust Grade C
Steel where all mill scale has flaked off and complete rusting has taken place.
Rust Grade D
Steel where pitting and complete rusting has occurred.
Surface cleanliness is categorised into four grades.
Brush-Off
Commercial
Near-White Metal
White Metal
Another surface quality is roughness or profile. This is the etched surface or “tooth” obtained by abrasive striking the metal.
Surface profile affects the ability of the surface to allow protective coatings to adhere. It is important that the surface roughness be sufficient for good paint adhesion.
Surface profile can be measured by several field methods. The best method of ensuring a specific profile is to specify the type and size of abrasive to be used for blasting.
Degree of surface cleanliness and specific profile on any job are established by the coatings of the manufacturer.
Once abrasive blasting needs are defined, equipment can be selected, personnel trained and procedures initiated.
Equipment used for surface preparation must be efficient, dependable, and easy to operate. Information for training and operation must be accurate and up-to-date.
