Coating on Concrete, Wood and Plastic

The bottom line

There is no second choice for coating measurement on non-metallic substrates: ultrasonic only. This judgement precedes any instrument selection — because magnetic and eddy current both fail here.

Which scenarios fall in this group

Scenario	Substrate	Coating system	Dongru model
Floor coating	Concrete, cement mortar	Epoxy, polyurethane, waterproof	DR200A (40-1200 μm)
Wood finish	Wood	Primer + topcoat, often multi-layer	DR200B (18-1000 μm)
Plastic-part painting	ABS, PC, PP etc.	Appearance paint, UV coating	DR200B (small probe fits curves)
Glass coating	Glass	Functional, decorative coating	DR200B

Standards and citation discipline

The standard basis for this group is ASTM D6132-13(2022) (standard test method for non-destructive measurement of applied organic coatings using an ultrasonic coating thickness gauge) and the ultrasonic clause of ISO 2808:2019; in the SSPC system it corresponds to SSPC-PA 9.

Where it is harder than on metal

Substrate acoustic properties vary widely: concrete has aggregate and pores, wood has grain and rings; velocity and attenuation differ;
Weak echo: low-impedance substrates give small interface echoes, demanding more from probe contact and coupling;
Uneven surface: rough concrete and wood grain reduce probe contact area;
Moisture effect: concrete and wood moisture state changes velocity; record and keep measurement conditions consistent.

Accordingly, industry practice for this group usually requires more spots, recorded environmental conditions, and repeated measurement at the same member averaged.

Layer measurement: the advanced capability on non-metallic substrates

Wood finishes and floor coatings are often multi-layer (primer + midcoat + topcoat). A normal gauge gives only total thickness, but a layer-capable gauge can read layer by layer, to judge whether a layer was missed. Dongru DR200A supports up to three-layer measurement.

State the capability boundary clearly

DR200A upper range is 1200 μm (about 47 mil), DR200B is 1000 μm (about 39 mil). That covers routine floor coatings, wood finishes and plastic-part painting, but not thick-film high-build coatings and polyurea ultra-thick soft coatings. State this boundary in external descriptions.

Common misconceptions

“Slip a steel plate under it and use a magnetic gauge” — you measure the coating on the steel plate, unrelated to the part.
“Ultrasonic thickness gauge and ultrasonic coating gauge are the same” — the former measures metal wall (millimetres), the latter coating thickness (micrometres); probe, algorithm and standard all differ.
“Concrete moisture does not affect measurement” — moisture changes velocity and biases the reading; record and unify conditions.

Frequently asked questions

Can I use a magnetic or eddy-current gauge on concrete, wood or plastic?

No. Concrete, wood, plastic, glass and fibreglass are neither ferromagnetic nor conductive, so magnetic and eddy-current methods are ruled out. Ultrasonics is the only applicable path.

Why are ultrasonic readings on non-metals less stable than on metal?

Echoes in low-acoustic-impedance materials are weak, so probe contact pressure and couplant amount affect stability more than on metal. For a reliable result, average several measurement points.

How does the environment affect the reading?

Moisture changes in concrete and wood shift the sound velocity. Measure under the specified condition and record the environment and time in the report, so the measurement can be reproduced.

What is the thickness limit for portable ultrasonic coating gauges?

They top out around 1000–1200 μm. High-build coatings and polyurea-type ultra-thick coatings fall outside the reliable band and should not be promised as applicable.

What actually causes most disputes on this application?

Not the reading itself, but whether the measurement condition was recorded and reproduced. Writing coupling, environment and capability limits into the inspection plan matters more than the gauge model.