Coating thickness gauges - Engineering Discussions - IET EngX - IET EngX
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Coating thickness gauges

Hi all,

A common issue on our clients' projects is quality control of the factory finish on outdoor equipment, which frequently either leads to snagging callbacks and/or early issues with corrosion. In addition one of our clients has seen fit to engage with a cheaper supplier whose quality control is even less trusted than usual. We are therefore looking into site acceptance (or, even better if it can be arranged, factory acceptance) testing to verify the finish and so raise repair before corrosion sets in.

It looks as though non-destructive ?ultrasonic? testers are readily available, which is great. However, the most common defect is on radiators either on the "sharp" edges where the front/back plates meet, or deep in the cavity between the fins. I am not sure if the most common devices will be happy with the surface of the edges, and it looks like they will struggle to get into the deeper parts.

Would you recommend a guage or test protocol for this and are there any common gotchas in their use?

If it helps, the majority of items are powder coating on steel and we are aiming working towards a C4/C5 specification.

Thanks in advance!

  • On flat steel a magnetic pull off test is possible and there is an equivalent that is really a small metal detector on a stick. But all these, like the ultrasound - which actually is better looking at non-metals of different density, need a reasonable area of contact, and checking the coat on a knife edge is really  tricky - and sadly it is a place the powder may not stick.. The other method I have seen but on military vehicles is to insist on a multi layer coating process where the lower layers are a really bright reveal colour that is shines through  if the top coat is either damaged or just too thin. Probably these are beyond the commercial budget. No chance to galvanize I suppose ?  far nicer as it tends to self heal odd scratches.


  • Thanks Mike. Multiple colours is not a bad shout; I will see if that can be worked into the test plan if it's not already.

    Can the pull-off test be reliably carried out in the depths of a radiator fin?

    Testers seem to be quite long "pens"... is this a technical requirement or are there probes oriented differently out there?

    Galv coating isn't likely to be an option, at least commercially.

    (FTAOD, it's not strictly a knife edge, just a narrow edge where two plates of a few mm meet. But I think you know that)

    In the absence of non-destructive tests, I have considered leaving areas of tape to be removed and measured with a retouch on site. But one suspects that these would be subject to additional attention during paint application, so probably not particularly effective.

    I am of an electrical bent though so could easily miss the bleedin' obvious when it comes to paintwork!

  • I suspect that none of these are easy inside a radiator fin...