This is the eddy current test on my 79 Piper Warrior per the recent wing spar airworthiness directive. The inspection is to discover cracks in the lower wing spar that can cost over $12,000 per wing to fix. My airplane passed.
The process is to have your mechanic remove the bolts then clean the holes with a non-abrasive method such as a scotch pad. Clean bolt holes will prevent the machine from registering any irrelevant crud as a crack. After the inspection, new bolts are installed and torqued. Be sure your mechanic removes the bolts carefully so as to not introduce nicks in the bolt holes which could cause your plane to fail the inspection.
There is some work required inside the cabin to remove and replace the bolts. After this AD came out Piper issued a service bulletin pertaining to the very bolt washers that were part of the replacement kit. Be sure your A&P addresses this before installing the bolts, otherwise you’ll be revising this area within the next 100 hours.