By far the most effective method for removal of mill scale, rust and old coating by using abrasives such as sand, grit or shots directed at high velocity against the surface. Prior to blasting, steelwork should be degreased and all weld spatters removed. Weld seams and sharp edges should be ground down. This is because paint coatings tend to run away from sharp edges, resulting in thin coatings and reduced protection. The grade of blasting suitable for a particular coating specification depends on a number of factors, the most important of which is the type of coating system selected. There are four commonly used grades for blast cleaning, and the approximate equivalents between the various International Standards are as follows