Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

5.13.2 Surface Coatings
Surface coatings are added to protect a surface from corrosion or to enhance
its appearance. Thin films, such as oxide layers, anodizing layers or
electroplated finishes, and the slightly thicker coatings of paint or lacquer are
usually well bonded to the surface. Quality of bond may be compared to the
uncoated reference block by a simple transfer value. Even a slight loss due to
the coating may be preferable to removing the coating and trying to inspect
on the rough surface it hides.
When thickness testing is done on a painted surface the paint thickness can
add error to the reading. For example:
A nominal 25mm steel plate has a cellulose paint coating of 0.5mm. V steel =
5980m/s, V paint = 2600m/s.
If a digital thickness meter is calibrated on a 25mm thick piece of the steel
plate without the paint coating and then placed on the painted surface an
error will occur.