If the steel shot or steel grit you are using is too hard, it may disintegrate on impact or cause damage to the surface, while if its too soft, it may disfigure in shape, on impact, and not be much use at all. Both extremes are a waste of time , and of course a waste of money. Somewhere between these extremes is the optimum harness abrasive for your application.
What is Hardness?
Hardness is a resistance of metal to plasting deformation - usually by indentation. This term may also refer to a metals stiffness, resistance to scratching, abrasion or cutting etc. It is the property of a metal which gives it the ability to resist being permanently deformed, bent or broken when an external load is applied.
How is hardness measured?
The most common hardness test applied is the Rockwell Hardness Test, This test is a hardness measurement based on the overall increase in depth of impression as a pre-described load is applied to the surface of the metal.