The coating can affect the ground bond. I always try using unpainted / unanodized fasteners because of that. At the very least take a wire brush to the fastener to remove any non-conductive material.
Powder coat systems can apply a very thin layer, invisible to the eye that can pervent any conduction, only 1 -2 moleculars thick.. If you are use something that has been powder coated , take a hot soldering iron, or small pencil torch and heat the area to remove the thin layer.
Learned this the hard way had units failing UL and CSA testing after changing to powder coat system.
Suggestion: Notice that the power panels use a special ground connection to avoid this problem. Perhaps it may be better to use as special ground connection to make sure that the ground is good.
