If microscopic pin holes are of concern, in addition to examining the surface for water leaks while the vessel is charged with water, the surface should be examined under black light after draining for yellow fluorescent dried water spots, evidence of a leak.
For maximum leak spot visibility, examine as soon as possible after leak spots have dried, since fluorescent indications will gradually fade. When inspecting, the area should be darkened to maximize contrast between the surface and glowing indication of a leak.
Fluorescent additive is a highly concentrated additive for hydrostatic leak testing. It adds fluorescent color to the water. Water leaking through defects to the exterior surface will carry fluorescent color for more positive flaw detection when the surface is viewed under black lights. While still wet, leaking water fluoresces blues after drying, the water spots fluoresce yellow.