DFT readings

[engineerpiep]

Sherwin Williams Zinc Clad II has a time to recoat of 18 to 48 hours depending on environmental conditions and a time to cure of 7 days.
Can I get an accurate DFT reading after time to recoat using a positect dft gauge?

Also I have a paint system spec that has Zinc Clad II, Dura-plate 235 and Hi Solids Polyurethane top coat. Should I be able to apply the next coat of paint after the time to recoat on the first layer of paint?

 

[TomDOT]

You can get an accurate DFT reading as soon as the coating is firm enough that the prope doesn't deform or dent the coating. Often for an IOZ this is even before you can recoat.

Recoating ethyl silicate IOZ early is BAD. It needs to absorb water from the atmosphere, AND release the ethanol which is being replaced with the water. I've seen some really nice, big "air pocket"* blisters when IOZ was recoated too soon.

Before recoat, you should be verifying cure with an MEK rub test. Typically this means a result of 4 or 5 using ASTM D4752.

Various "coin" or "scratch" tests for IOZ cure are much less accurate, thus your risk goes up. And D4752 takes maybe 2 minutes to complete. To be totally sure on cure, you need to do a GC procedure** - but that's typically for failure investigation, not QC.

*Okay, more like ethanol vapor pocket, once the girder was sitting in the sun and heated up.

**Scrape off some IOZ. Weigh it into a vial. Add a known excess of water and spike with a known amount of IPA as an internal calibration standard. Seal. Heat up the vial in an oven to force the conversion reaction. Cool, run GC and measure the ethanol produced. Compare against an unreacted IOZ sample, and if you are really thorough, against a "fully cured" IOZ sample.