Dry film thickness arguments on site usually come from one misunderstanding: someone treats a single gauge reading as a pass or fail. SSPC PA 2 is built on averages, in a defined hierarchy, and knowing it settles most disputes before they start.
The three levels
Gauge reading. One instrument reading. On its own it means nothing for acceptance.
Spot measurement. The average of at least three gauge readings taken within a circle roughly 4 centimetres across. This is the smallest unit that carries any acceptance meaning.
Area measurement. The average of five spot measurements taken across each 10 square metres of coated surface.
The acceptance rules
The mean of the spot measurements across the area must be at least the specified minimum thickness. Individual spot measurements are commonly permitted between 80 and 120 percent of the specification unless the project states otherwise. Worked example: a specification of 200 microns means spots between 160 and 240 microns are acceptable individually, provided the area average meets or exceeds 200.
So a single low gauge reading is not a rejection. Three low readings averaging into a spot below 80 percent is a finding at that spot. A pattern of low spots dragging the area average below specification is a genuine failure requiring additional coats.
Getting the measurement right
Calibrate the gauge and verify against certified shims on the actual substrate profile, because blast profile affects readings on ferrous substrates. On profiled steel, agree the base metal reading correction with the inspector before starting rather than after a dispute. Take measurements after the coat has cured sufficiently, not on soft film.
Why over thickness matters too
Excess film is not automatically better. Over application causes solvent entrapment in thick epoxies, sagging on vertical surfaces, cracking in some intumescent and zinc silicate products, and in fireproofing it can indicate mixing or application problems worth investigating. Upper limits exist for good reasons and should be enforced, not treated as generosity.
Documenting for handover
Record spot and area measurements with locations, coat by coat, alongside environmental readings taken during application. That package, combined with the preparation records described in our Sa 2.5 guide, is what a client quality file should contain.
MENA Industrial Supplies supplies coatings with clear specified film builds and data sheets that state minimum and maximum thickness per coat.