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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!
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?
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.
I hope this helps a little
Arturo mentioned that plastic shims can protect your sensor from high substrate temperature and rough textures which is true but for this you have special probes which can go to higher temperatures also it may protect the probe against wear and tear due to rough surfaces but I never used a shim in between ones taking readings. It might be helpful when you have soft coatings and use a shim or calibration foil for taking reading taking in account you have to record this in your raport.
Also stated is that as per ASTM D709, DFT is the distance from the highest peaks of the substrate up to the surface of the coating which may be correct but I prefer to use the ISO 19840 for this where you calibrate your gauge on smooth calibration plate and use a correction value for the blasting profile. calibration on blasted profile is a crime and absolutely gives you incorrect readings.
Your interpretation of taking readings in DFT's is thus incorrect.
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