Abrasive Blast Cleaning:

By far the most effective method for removal of millscale, rust and old coatings is abrasive blast cleaning using abrasives such as slag, grit or shot under high pressure. The grade of blasting suitable for a particular coating system depends on a number of factors, the most important of which is the type of coatings being used.

Prior to blasting, steelwork should be degreased and all weld splatter removed by grinding. Salts, grease or oil if present on the surface will appear to be removed by the blasting process, but this is not the case. Although not visible, the contamination will be present as a thin layer and will affect the adhesion of the subsequent coatings. Weld splatter is almost impossible to coat evenly and all too often is loosely adherent and it is common cause of premature coating failure.

The surface profile obtained during blasting is important and will depend on the abrasive used, the air pressure and the technique of blasting. Too low a profile may not provide a sufficient key for the coating, while too high a profile may result in uneven coverage of high, sharp peaks leading to premature coating failure, particularly thin film coatings. In general fine to medium profiles  typically 30-50 microns are used for low build solvent based products, whilst coarse profiles, typically 75 microns and above are recommended for high build and solvent free systems.

The following table gives us a brief guide to a typical roughness profiles obtained using various types of abrasive:
 

There are four internationally recognized standards for cleanliness of surfaces produced by abrasive blasting:

Sa1:        Removal of all contaminants with the exception of toughly adhered residues millscale rust and coatings.

Sa2:        At least 65% of surfaces is free of all contamination and residues.

Sa2.5:     Blast cleaning until at least 95% of surface is free from rust, millscale and coatings.

Sa3:        Complete removal of rust, millscale and coatings.