Coating Thickness at Four Stages of Production

1. Incoming: judging supplier consistency

Subcontracted and bought-in parts arrive, and the question is whether the batch can be accepted. Sample by batch and look at the distribution: is the mean centred in the tolerance, is the spread too wide.

A wide spread usually means the supplier coating parameters are unstable, which is more worrying than a single out-of-tolerance reading.

2. In process: controlling the spray

During coating, the question is whether this pass is right. Pressure, distance, gun speed, number of coats – change any one and the thickness changes.

This stage has to be fast: a hand-held gauge on the surface gives a number in seconds, the operator checks it without waiting for quality, and nothing needs archiving, but limits and alarms do.

3. Acceptance: producing compliant data

The last gate before delivery, run to the national standard or the customer agreement: sampling ratio, point locations and decision rules all specified.

This data is retained, the instrument must be within its calibration validity, and the report must trace back to instrument serial number and calibration record.

4. Maintenance: estimating remaining life

Coatings on equipment in service – bridges, tanks, pipe – thin year by year. Re-measure the same points on a schedule, read the thinning rate, and plan the recoat from it.

Fixed points and a continuous record are the whole point. A different person measuring a different spot produces data that cannot be compared.

5. What all four require

Confirm the substrate: ferrous or non-ferrous, and do not switch the mode by accident.

Calibrate the same day: on a foil over the actual substrate, not on the bench before you walk out.

Keep points off edges, weld beads and corners, where the probe cannot sit flat and reads low.

Summary

Coating measurement looks like touch-and-read. In fact each stage answers a different question, and the instrument is only right when the question is clear.