Most fiber-reinforced plastics have undulating surfaces due to the weave of the fibers. To bond a straingage on such surface , it is necessary to first create a flat smooth surface. One way is to abrade the surface with silicon carbide paper till the corrugations are removed fully, but this is often not allowed as the possible removal of fibers in this process is likely to alter the structure significantly, and the strain results could be questionable.
An alternative method would be to fill the uneven surface with M-Bond AE-10 adhesive, which can then be abraded flat to get a suitable surface close to the top level of the fiber test specimen.

In this video, the preparation of the surface of the FRP is shown in detail, with the application of M-Bond AE-10, which is sanded down to the top level of the test part without disturbing the underlying fibers. The First step in surface preparation is to degrease the surface with CSM-3 degreaser as shown. Some plastics may react to the CSM-3 degreaser. As a precaution, it is best to try applying the degreaser to a corner of the test specimen, to check for any reaction.An alternative solution is to use GC-6 from Micro Measurements to degrease the part.

Once the corrugated surface is filled and sanded , the procedure for bonding straingage is completed as per standard procedures laid down in instruction bulletin B-127 of Micro Measurements. These steps are illustrated in detail in the video, using M-Bond 200 adhesive to bond the strain gage, which requires about one minute of thumb pressure to set . Normally, after the Thumb pressure is removed, about one or two minutes are allowed to lapse before removing the gage-handling tape.
In this video, the lead wires are shown being soldered directly to the large copper-coated solder tabs of the Micro Measurements CEA series strain gage. However since the FRP is a poor conductor of heat, care should be taken not to over-heat the solder tabs, especially for smaller straingages with much smaller solder tabs. In such cases, it would be preferable to install strain gages with pre-attached lead wires, such as the C2A, C4A C5K series of strain gages, or the EA-series with option P, or the CEA series gages with option P2. All these gages are supplied with pre-attached lead wires of 10 feet, and soldering of the lead wires to the gages can be avoided.