The "industrial" version of this is an ultrasonic probe which is passed
over the surface. (Sort of like the OB/GYN's ultrasound device).

jsmith wrote:
The "tap test", as used on fiberglass homebuilts, is used to detect
voids (air bubbles) in the epoxy/glass matrix. One usually uses a
Quarter to gently tap the surface. If there are any voids, the
difference in sound will be noticeable.

George Patterson wrote:
How long do you think it will take to do this on an Airbus rudder?