If you can attach a suitable ground, and immerse the rest of the part, you can measure conductance between metal and surrounding water. Debatable whether this means anything over any period of time, or in the presence of contaminants that may break down the coating...
The best way, as alluded by IRstuff, is to let the part sit outside for a year or two, and look for bubbles under the paint.
The rubber lining industry use a Spark Tester - - - a high wilvoltage "wand" is waved over the surface and the 2 inch long spark will instantly show any holes in the coating.
Corrosionman
Great advice by all. Consider, too, weight gain by your film. This would require smaller test samples, and an analytical scale with capability to 0.0001 gram. Best wishes.
Can you deposit the coating on a different substrate? There are machines that can determine the moisture permeability of polymeric films. If you can deposit on something that was highly permeable to moisture like say polyester, you can then get an actual water permeation rate (weight of water / thickness of film / time)
For coatings, (high voltage) spark testing is typically only used prior to service. Immersion fluids frequently increase conduction enough to allow burn-through, damaging what was previously a perfectly good coating.
Tiny pinholes in a coating are called "Holidays." They can be detected with low and high voltage holiday detectors. Be sure to talk to the paint supplier and holiday detector manufacturer to select the correct type of detector and that it is used correctly. If too high of voltage is used, the detector can damage the coating.
Be wary that this testing is sometimes seen as excessive by painting contractors and can cause extra work by making sure that there is acceptable coverage. There are also ATSM Standards that concern holidays.
NACE Certified Level 3 Coating Inspector
8 Years of Painting Inspection Experience
