Visual Inspection is often underrated, but it is one of the most established and reliable methods of primary inspection (when deployed by a professional who knows what they're looking at). 

 

Tap-testing, in many ways the original form of sonic-testing, is also often overlooked by other NDT companies.  However, when used correctly, it can be an excellent method to initially identify defects as well as confirm, in conjunction with ultrasound, the type of defect.  With a former career as a yacht surveyor and boat builder, Ben Pierrepont and his team use visual and tap-testing as a precursor to modern NDT techniques such as ultrasound.

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Dye testing on metal structures such as keel bolts and rigging, is also a valuable NDT method