Document paint-thickness gauge checks and readings with PosiTest DFT

The short answer: pick the PosiTest DFT Ferrous if every panel you measure is steel, or the PosiTest DFT Combo if you also measure aluminum or other non-ferrous metal panels. Before measuring, confirm the gauge reads correctly with an operational check. Write down every reading yourself, because the DFT does not store readings and clears them when it powers off. A DFT reading is the total thickness of all coatings over the metal. It cannot tell you how much clear coat is present or how much can safely be polished away. The gauge does not measure over plastic or fiberglass, so those panels need a different instrument.

Choosing between the DFT Ferrous and DFT Combo

DeFelsko makes the PosiTest DFT in two versions. The difference between them is the type of metal under the paint. For automotive paint, DeFelsko describes three measurement methods: magnetic measurement for steel, eddy-current measurement for other metals, and ultrasonic measurement for non-metal substrates. The two DFT versions cover the first two methods.

Model	Substrates covered (per DeFelsko)	Display indicator	Typical automotive use
PosiTest DFT Ferrous	Nonmagnetic coatings on steel	Shows F	Steel body panels only
PosiTest DFT Combo	Nonmagnetic coatings on steel, plus nonconductive coatings on aluminum, brass and similar metals	Automatically shows F or N for the detected metal	Mixed fleets or vehicles with aluminum panels
Neither DFT model	Plastic, fiberglass and other non-metal substrates	Not applicable	Bumper covers and composite panels need an ultrasonic gauge such as DeFelsko's PosiTector 200

If you are unsure whether a panel is steel or aluminum, the Combo is the safer choice. Its F or N indicator also records which measuring principle the gauge used, which is useful documentation in its own right. A Ferrous unit gives no valid reading over aluminum.

Key specifications that affect your records

• Measuring range: 0 to 1000 µm.
• Stated accuracy: ±(2 µm + 3% of the reading).
• Units: selectable between mils and µm in the menu. One mil equals 25.4 µm.
• Running average: up to 99 readings.
• Memory: no stored readings and no printing. Readings are lost at power-off.
• Supplied items: a calibration certificate and plastic shims for a quick operational check.

The accuracy figure grows with thickness, so include it when you compare readings. For example, at a reading of 120 µm the stated tolerance works out to ±(2 + 3.6) = ±5.6 µm. Two readings a few microns apart on the same panel may therefore fall within the gauge's own tolerance. That small a gap is not, by itself, evidence of a difference in the finish.

Checking the gauge before you measure

The PosiTest DFT manual (version 4.0) describes three separate actions. They do different things and should be recorded as separate entries.

• Calibration certificate: the gauge ships with one. Record the gauge serial number and certificate details on your worksheet so the readings can be traced to a specific instrument.
• Operational check: place the gauge on a supplied plastic shim laid over bare metal and confirm the reading is reasonable for the shim's marked value. DeFelsko describes these shims as non-certified. They show the gauge is working, but they are not certified thickness standards.
• Zero adjustment: set the gauge to zero on an uncoated part. Use bare metal of the same type as the panels you will measure. Note its curvature and surface condition, because an unrelated reference may not behave the same way.
• One-point adjustment: adjust the gauge to match a reference of known thickness. Record exactly which reference you used and its stated value. If the record must show traceable accuracy, use a certified thickness standard rather than the included shims.

Write down whether you only checked the gauge or actually adjusted it. A check leaves the gauge's settings unchanged, while an adjustment changes how the gauge reads from that point on. Anyone reviewing the record later needs to know which one happened.

Taking readings on a panel

• Confirm the unit setting (mils or µm) and write it at the top of the sheet.
• Place the gauge flat on the painted surface and hold it steady until the reading flashes.
• On a Combo, note whether the display shows F or N. An unexpected letter suggests the panel material differs from what you assumed.
• Record each individual reading immediately, together with the panel and its location on the panel.
• Take several readings per location. The running average is convenient, but write the individual values down too, because the spread between them matters.
• Transcribe everything before switching the gauge off.

A worksheet layout you can copy

DeFelsko does not supply a printable reading log for the DFT, so you need your own. The layout below can be pasted into a spreadsheet or word processor and saved as a PDF. Fill in the header once per session and add one row for each measurement location.

SESSION HEADER
Gauge model:            PosiTest DFT [Ferrous / Combo]
Serial number:
Manual version:         4.0
Calibration cert ref:
Units:                  [µm / mils]
Date / operator:
Conditions:             (temperature, surface cleanliness)

INSTRUMENT CHECKS
Check type        | Reference identity       | Known value | Displayed | Adjusted? (Y/N)
Operational check | Supplied shim #          |             |           |
Zero              | Bare-metal ref, describe |      0      |           |
One-point adjust  | Standard ID / certified? |             |           |
Reference notes:  (curvature, surface condition)

PANEL READINGS
Panel | Location | Substrate evidence | F/N shown | R1 | R2 | R3 | Mean | Range | Notes

Worked example (hypothetical numbers)

The following readings are invented to show the arithmetic. Suppose three readings at one spot on a hood are 118, 121 and 120 µm. The mean is (118 + 121 + 120) / 3 = 119.7 µm. The range is 121 − 118 = 3 µm. Record all three values, the mean and the range in the panel row before turning the gauge off. These figures describe only those three readings. They do not indicate how much of the 119.7 µm is clear coat, whether the panel has been repaired, or how much finish can be removed.

What the record cannot tell you

Every PosiTest DFT reading is the total thickness of all coatings stacked on the metal: primer, base coat, clear coat and any refinish layers combined. The gauge does not separate these layers. Neither the total nor a comparison between panels gives you a polishing allowance. DeFelsko's discussion of measuring individual layers and plastic or fiberglass panels applies to the PosiTector 200, a different instrument. If your decision depends on clear-coat thickness or on readings over a bumper cover, the DFT record cannot answer it. In that case, use an instrument designed for that purpose.