Proper Surface Roughness
When abrading harder materials (many tool steels for example) the use of 400 grit silicon carbide paper tends to “polish” the surface rather than causing a uniform roughness conducive to proper adhesion of the gage.
On the harder metals, we suggest starting with 220 grit, finishing with the 320 grit. The increased roughness allows the adhesive to better bond to the surface especially when the anticipated strain levels are high.
Final surface finish should be between 63-125 RMS (1.6-3.2 µm) for general stress analysis. For High elongation > 250 RMS (>6.4 µm). For precision transducers, with their relatively low strains, the surface should be 16-63 RMS (0.4-1.6 µm).
Surface Cleaning Supplies: http://www.vishaypg.com/docs/11008/surface-cleaning.pdf
Surface Preparation for Strain Gage Bonding: http://www.vishaypg.com/docs/11129/11129B129.pdf