1. Pick one unit from the start: mils or microns

Powder coating thickness refers to the cured dry film thickness (DFT) left on the part after the powder melts and hardens, not the loose powder you see right after spraying. It is stated in two units:

• Microns (µm): one thousandth of a millimeter, most common outside the United States.
• Mils: one thousandth of an inch, common in the U.S. One mil equals about 25.4 microns.

Choose one unit and use it consistently across your drawing, your purchase order, and your inspection sheet. Mixing units is the most common source of specification errors.

2. Start with typical thickness ranges

Most standard polyester or epoxy powders on industrial sheet metal parts cure between 2 and 5 mils (50 to 125 microns). Within this working range, practical targets divide into three bands:

• 1 to 2 mils (25 to 50 microns): thin builds for parts with tight mechanical tolerances or parts that seat into close-clearance assemblies, where even 0.003 to 0.004 inch of extra material causes binding.
• 2 to 5 mils (50 to 125 microns): the standard range for most stamped parts, balancing corrosion protection, impact resistance, and fit.
• 5 mils and above (125 microns and up): heavy-duty or high-build coats for parts exposed to gravel, abrasion, or aggressive chemicals.

3. Factor in how the part fits and functions

Every mil of coating is a dimensional change: it shifts the coated surface inward by roughly that amount. On holes, threaded holes, sliding fits, and mating surfaces, an over-specified coating will cause binding during assembly. If a part bolts together, slides in a track, or seats into a precision housing, cap the maximum thickness at the same time you raise the minimum. This is exactly the kind of detail a manufacturer that both stamps and finishes parts can flag early, before the drawing becomes hard tooling.

4. Match the coating to the environment it will live in

Indoor parts with light handling can often get by with less than 3.5 mils. Components exposed to salt spray, ultraviolet light, fertilizers, or frequent impact benefit from 4 mils or more. For outdoor architectural and automotive exterior parts, common practice targets a range of roughly 60 to 120 microns with a clearly defined minimum. The most reliable specs tie that minimum to a corrosion test result, such as a required number of hours in salt spray, rather than to a bare number.

5. Write it as a range with a minimum, not a single number

A single thickness number is almost impossible to hold on a real part. Edges and corners attract more charged powder and coat thicker, sometimes up to double the film on flats, while recesses and blind holes coat thinner. A practical specification includes:

• A target range, for example 2 to 4 mils.
• A minimum at critical features, for example 1.8 mils minimum on edges and flats.
• Explicit callouts on critical fit areas, threaded holes, and grounding points.

This gives the finisher room to work while protecting the performance you actually care about.

6. State how the thickness will be measured

Specify the measurement method up front so there is no argument later. Professional shops verify cured thickness with non-destructive dry film thickness gauges:

• Magnetic induction gauges on steel and other ferrous substrates.
• Eddy current gauges on aluminum and other non-ferrous metals.

Readings should be taken at multiple points across flats, edges, and recesses, then averaged, with points kept away from welds and sharp corners where readings distort. Referencing a standard such as ISO 2178 or ASTM D7091 means both you and the supplier know exactly what is expected.

7. Work with a supplier that builds the part, not just finishes it

The cleanest way to remove guesswork is to bring the coating spec into the same conversation as the stamping geometry. A manufacturer that both stamps and finishes sheet metal parts can flag fit and edge-coverage issues before they cost you a rework, and can tune pretreatment and spray settings to hold thickness consistently across a full production run. That is the advantage of working with a single factory rather than juggling separate tooling, forming, and coating vendors.

Conclusion

Specifying powder coating thickness is not about picking a random number. It is about balancing the environment the part lives in, the fit it must keep, and the measurement you can actually verify. Define a range, set a minimum on critical features, choose one unit, and agree on the measuring method. Put those four things on paper, and the rest of the process is straightforward for any experienced finishing partner.