DIY Steel Blackening with Vinegar – Why the Uneven Finish?

[asto]

Hi All!
Trying to blacken steel parts with vinegar, l resulted in uneven surface colouring.
Can't figure out what it was that may have done wrong.
Is there someone here who has succeeded blackening parts with this method?

[Lee Roberts]

Hey Delta!

Yep, a few folk here have experimented with vinegar blackening / forced patina methods over the years, uneven colouring is usually down to one or more things from what i've seen:

Surface prep, any oil, fingerprints, polishing compound, or mill scale left on the steel can stop the reaction taking evenly, always remember, the cleaner the metal, the better the finish. Acetone or brake cleaner is a solid choice you can use beforehand and will help a lot.

Steel composition, different steels react very differently Mild steel tends to work better than some alloy steels or stainless.
Oxide layer inconsistency, if parts have been heated, welded, or previously oxidised unevenly, the vinegar can react "patchily".
Solution strength / contamination, old vinegar or reused solution can give inconsistent results, always use fresh juice.
Temperature, slightly warming the part or solution can help the reaction become more even, nothing crazy just take any chill out the metal. I cant remember if this one works for Anodising too !?!

A lot of people doing the “vinegar blackening” method are actually using vinegar and hydrogen peroxide with salt, which tends to create a more aggressive oxidation layer before oiling and/or waxing the finish.

One thing I’ve noticed myself is the finish often looks terrible until you wipe the loose oxide off, repeat the process a few times and then seal it with oil or wax. That’s usually when it suddenly starts looking much more uniform and deeper black rather than patchy grey/brown.

Tell us what type of steel your using, what prep you did and maybe a photo of the finish you got.

Be interesting to see what methods others here have had success with