The required surface condition depends on the type of test and load used.
Usually a ground surface is sufficient for macro hardness testing, and sometimes no preparation is required. Micro hardness testing requires a polished or electropolished surface. It is important that the borders/corners of an optically evaluated impression are clearly visible.
In the micro hardness range, the lower the loads used during hardness testing, the greater are the requirements for surface preparation. This can be performed mechanically, chemically, or electrochemically. It is important that no change of surface properties is induced in the specimen during preparation due to heating or cold working.
Deformations introduced during cutting and grinding need to be removed by polishing down to 6.0, 3.0, or 1.0 μm, depending on the test load. For very small loads, less than 300 gf1, the surface needs to be completely free of deformations, and the specimens require oxide polishing or even electrolytic polishing to obtain a completely damage-free surface. One should also take into account that soft and/or ductile materials (i.e., for HV less than 120-150) are more sensitive when it comes to introducing preparation artifacts.
In general, however, it can be said that the variation of the measured hardness result relates directly to the quality of the surface preparation. Thus, it is a good idea to consider the trade-off between surface quality and test result variation before deciding on an inferior surface preparation.
Reference: 1 Metallography and practice, George F. Vander Voort