Why DFT matters
A coating system is qualified at a stated thickness range, and both directions of error cost service life. Too thin means premature barrier failure — the film's permeation resistance scales with thickness, and thin spots become the first corrosion sites. Too thick is not a bonus: excess thickness traps solvents, generates internal stress, and promotes cracking and delamination, particularly on edges and in cold-cure conditions.
How it is measured
On steel, DFT is measured non-destructively with magnetic or electromagnetic gauges; on non-ferrous substrates, with eddy-current instruments. Measurement practice — how many readings, where, and what statistical rule accepts an area — comes from the governing standard cited in your specification (SSPC-PA 2 is the most common reference in North American work). Spot readings and area averages are judged separately: a specification that only quotes an average invites thin spots.
The specification errors we see most
A DFT clause that names a single number with no range and no measurement standard; average-only acceptance with no minimum spot value; and thickness specified without reference to the measured surface profile — profile peaks consume part of the film, so the effective DFT over peaks is what protects the steel.