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Can't get good Black Oxide on reworked parts

We have a batch of automotive gear to rework by black oxide. For this batch, we have reworked several times. We can see no rusty spots, but some light yellow spots, and we cannot get a uniform black.
We remove the old coating of Autophoretic 7005, and control the temperature of the tank solution between 143-147 C, 20 minutes of treat time. What can we do then? Thanks.

Jessica Wang
Chemist - Suzhou, Jiangsu, China
April 29, 2010

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I have tried a host of acid strippers over the years and all of them seem to leave some sort of residue behind that makes parts difficult to re-blue after use. Every gunsmith I have ever known has told me the same story. I would first check the blueing batch by polishing one of the parts by hand with light sandpaper or with a buffing wheel and then degreasing it and running it through the tanks. If it blues it's something left over from the stripping process. If it does not blue then it is either the blueing bath or the material you are trying to blue. Trying to blue some generic 1018 or 4140 will tell you if the bath is working. If it does turn out to be the acid stripping bath you might try blasting the parts with organic compound like walnut shell media [on eBay or Amazon] media. Time consuming but it should remove the residue left behind by the acid without changing the dimensions or weight of finish on the parts. It is better NOT to strip parts if mechanical polishing is not an option. Simply degrease them and run them through the blueing process.

Rod Henrickson
gunsmith - Edmonton, Alberta, Canada
May 5, 2010