In reality the coating on ships is applied in all weather conditions and to metal that may already be rusty. The rust under the coating attenuates ultrasound from reflecting from the rust-coating boundary because of air being present and/or because of the very course rust surface. In addition some naturally coatings attenuate US so much that no echo is produced (they contain tiny pockets of air) . Consequently ultrasonic methods are not appropriate.
The method that always works is the use of the magnetic properties of steel or iron. Magnetism passes through any non-magnetic coating, and just about 100% of ships coatings are none-magnetic and through air so a magnetic method is idea for measuring the thickness of ships coatings. There is one other factor, ships coating are usually quite thick so gauges using magnetic principles must in addition be able to measure up to say 8mm or 0.3". The Touchstone 2* manufactured by Class Instrumentation Ltd satisfies all of the above requirements.
ABS Classification Society Manufacturing Process Approved
ISO 9001:2008 Certification Number GB13487
IIMS Corporate Membership Number C508
NCAGE Number U0B22
NFPA Membership Number 2800960
FPA Membership Number 27135