My first thoughts is oil contamination. Are your carding wheels or wire wool thoroughly decreased ?
I use ferrobronze a lot and really like it . I believe there's a point, well before the metal can pit that the rusting can remove some of the black from the previous pass. Just like putting too much solution on (too wet) can do.
Chemical quality, I get a lot of mine from that auction site so given that most are 99% pure according to the label I would suspect that's good enough. I've always wondered what the other 1% is made up of though.
