Given that impact treatment is based on kinetic energy, it follows that selecting a large pellet will give a greater impact than a small one and vice versa. The initial size selection, therefore, is very important for the process. The abrasive sup- plied by the manufacturer is predominantly one size. The size range in the operation is known as the ‘operating mix’ and consists of the nominal size shot or grit and the worn particles (shot) or broken down particles (grit). It includes pellets that, during the operation, become polished, work hardened and conditioned during their life developing into an optimum cleaning, or peening tool. The operating mix provides a balance of particle sizes for impact and coverage. Impact is pro- vided by the new abrasive or larger size pellets to remove heavy contaminant and coverage is provided by the medium and smaller pellets to remove light contaminant and, more importantly, to give the final finish to the work processed. This smooth finish is aesthetically pleasing and minimises the amount of material required for subsequent surface coa- tings, whilst retaining the required adhesion factor. This balance between impact and coverage is important and can only be maintained by regular additions of new abrasive, preferably at the abrasive breakdown rate, by the wheel hour or by the golden rule of ‘little and often’. It is also necessary to return back to the system any abrasive losses or carry-out from the machine. As a result, the number of particles per kg thrown will be higher, coverage will be improved, cleaning will be faster and blast time will be reduced. Economics are thus achieved in energy, wear and tear of equipment, abrasive consumption and reduced maintenance. To ensure such benefits with cast iron abrasives, the specification should always be the largest particle that will not cause damage to the component being cleaned.