Among other pass times (sailing is better), I am and API licensed petroleum tank inspector - I test tanks
from 2,000 to 12,000,000 gallons.
Although thickness testers are very good for testing the general thickness of metal for structural evaluations, they will seldom find small pits - the kind that cause small leaks
. One reason is that the probe must pass right over the defect - tedious. The other is that the transducer is typically bigger than the pit and will often span the hole.
You should certainly have a calibration sample - a piece of steel of similar known thickness with similar paint. That will give you the accuracy... except for one thing....
The unit must pick the "best" echo and give a thickness. If the surface is rough it gets many to choose from. If you are having trouble getting a "lock" in a given area, that can be a sign of corrosion
You should be able to find lots of information on-line. Good luck. They are great tools, once you understand what they are.