Black streaks = zinc contamination, probably from some steel parts that were, or had been at one time, zinc plated. Sandblasting these parts blasts the zinc particles into the steel, making it much harder to remove.
Plate a steel dummy to plate out the zinc contamination, then sandblast, acid etch in your acid tank, then blast again.
And of course, only use distilled water in all your processes to avoid possible contamination.
The pH of OUR nickel solution should be approx 4.
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