Rust spots on drip line

Help!

I have "rust spots" along the drip line of my parts. The last area to dry has a dark colored spot on it as it comes out of the dry-off oven. We had this problem once before and never got to the bottom of it. Then it magically went away and now a year and a half later it has returned. We apply a zinc phosphate to the parts. Analysis of the area in question has shown that the spot is ON TOP OF the zinc phos. The spot is iron oxide. My washer has 3 stages of DI rinsing after the zinc phos. The conductivity of these stages is progressively lower with the final rinse being < 10 microsiemens. So I can't imagine anything being in the water that would cause this contamination to happen as the conductivity would shoot up, right? The parts go through a gas fired dry-off oven. No problems with the burner tubes have been found. Any ideas?

Dear David,

The problem is not your rinsing. For my opinion the problem attributed to the quality of the Zinc Phosphate coating. You must check the activity of the grain refine stage and the operation parameter of the Zinc Phosphate bath.