Hi Edward,
I recently joined the community, I have a little of information regarding your second question.
My second question is about taking DFT readings with a PosiTector gauge. One of the Inspectors is stating to not take DFT reading with a plastic shim. Is this correct practice?
- Although a DFT gage as the PosiTector will read coating thickness as soon as the sensor touches the coating surface, we need to be conscious of the factors causing interference with the response of the sensor.
- As per ASTM D7091, verification should be performed at least twice every shift on all gages regardless of the manufacturer name and model. When needed, an adjust (a physical action) needs to be performed by the operator of the gage in order to compensate the effect of the surface on the sensor correcting the error caused by surface profile (BMR)
- Not doing so can cause the gage to read a DFT different from the real one.
With my NACE level 1 and 2 training, I was always under the impression that the use of a plastic shim not only protects the DFT gages probe but also helps take away the peaks of the blast profile. So, when you take DFT readings you are getting the majority readings based on the valleys.
- Yes plastic shims can protect DFT sensors from high substrate temperature and rough textures.
- As per ASTM D709, DFT is the distance from the highest peaks of the substrate up to the surface of the coating.
- Hence, the shim placed on top of surface profile actually helps the gage to set a reference for the metal surface. So when we adjust the gage to read the correct thickness, the sensor will measure DFT from the highest peaks to the surface of the coating.
I hope this helps a little
Regards
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Arturo Escobedo
Monterrey, MX
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Arturo Escobedo
Twilight S.A. De C.V.
Monterrey
+528181151400;113
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Original Message:
Sent: 05-17-2023 10:58 AM
From: Edward Kurcz
Subject: Coating Repair to Substrate and DFT with or without plastic shim
Hi all,
I have two technical questions for you based on a job I am working on with multiple other inspectors. We are working with a DTM water-based coating.
After painting a blasted surface, if the coating is showing signs of failure from excessive milage the small repair procedure would be to DA sand the area down and apply a light coat after cleaning the sanded area. If the sanding goes down to the substrate, would you need to re-blast the area or would there still be a properly prepped surface for painting?
It is my understanding the sanding down to the substrate would not disrupt the prepped surface, but I am looking to see what others have experienced when doing this.
My second question is about taking DFT readings with a Positector gauge. One of the Inspectors is stating to not take DFT reading with a plastic shim. Is this correct practice?
With my NACE level 1 and 2 training, I was always under the impression that the use of a plastic shim not only protects the DFT guages probe but also helps take away the peaks of the blast profile. So, when you take DFT readings you are getting the majority readings based on the valleys.
Thanks in advance for your help!
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Edward Kurcz
Dallas, TX
+17082188556
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