Standard · Application and film thickness
ASTM D4138
Dry film thickness by destructive cross-sectioning (the Tooke gauge or PIG).
ASTM D4138 measures dry film thickness by cutting into the film at a precise angle and reading the cut under a lighted microscope. The paint inspection gauge (PIG) and the Tooke gauge are the instruments most inspectors know. The cut is made in place on the structure; no coupon is removed.
Why use a destructive test
It shows each coat separately when the coats differ in colour, it works on any rigid substrate, and it lets the inspector see the film in section. The price is a cut that must be repaired.
Reading it
The cutting tip sets the scale: on the common Dual-Measure reticle one division is 1 mil with the 1× tip, 0.5 mil with the 2× tip and 0.1 mil with the 10× tip, so the 10× tip suits thin films. Count the divisions across each coat and multiply by the value of one division for that tip. D4138 sets no number of readings: the parties agree where to cut and how to repair.
6 practice items in the members area mention ASTM D4138 (CIP 1: 1 · CIP 2: 4 · FROSIO written: 1).
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This page describes the subject of ASTM D4138 in our own words; it does not reproduce the standard. For the full text, buy the current edition from ASTM International. Checked on 2026-10-14; errors go in the corrections log. CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.