PosiPen magnetic thickness coating gauge - video

[Toolmaker51]

Don’t know if “holiday” is a generally used term, but in Naval terms it refers to gaps in the paint coating, big or small. They are looking for uncovered areas/pin holes in the paintwork.

That is accurate; though coating holidays in our esteemed USN detectable by eye, being some portion the width of a roller or brush. On new work holidays are prevalent on texture of welds and seams. Later, they'll weep tiny trails of rust, undermining the point behind surface protection. But they have a ready supply of additional paint chippers and painters.

Industrially, especially manufactured products, inspection methods for holidays is substantially smaller. The charged brush is a low voltage method, but not so visible in normal lighting. Company made hydraulic reservoirs for military aircraft tow vehicles, a step shaped box with MIG welded ends and fittings. Before powder coating, welds were 100 percented @ 30 PSI and sudsy water; ~8' of weld and 6-7 fittings. They also were examined visually for weld residues, pitting, voids etc. The same areas were sparked for holidays after olive drab paint. Powder paint is highly effective, but obstructions and pockets are challenges. Residues and pitting don't receive paint well, mainly being non-conductive impurities.
Adhereing to MIL-SPEC requirements keep a lot of fabricators out of that work; only a few prosper. Built in; not bolted on.
Achieves what ISO-9000 series certifications of "Model for quality assurance in production, installation and servicing." wishes it could. Real quality results, not empty 'assurance' via administrative clowns.