Surface Preparation

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The Surface Preparation Technical Community of Interest is a group of people (both AMPP members and nonmembers) who have come together for the purpose of furthering a common cause by sharing wisdom, knowledge, information, and/or data for or preparing surfaces in order to clean and/or increase adhesion to coatings and linings.
  • 1.  Coating Repair to Substrate and DFT with or without plastic shim

    Posted 05-18-2023 07:59 AM

    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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  • 2.  RE: Coating Repair to Substrate and DFT with or without plastic shim

    Posted 11-23-2023 02:03 PM

    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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  • 3.  RE: Coating Repair to Substrate and DFT with or without plastic shim

    Posted 13 days ago

    Hi Edward,

    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.

    Kind regards,

    Jan



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    Jan Wink
    Gorinchem
    +31651868499
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