I don't use the blackener on anything but pure plated parts, so this may not eb entirely applicable, but ...
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I'm having difficulty achieving a black finish on a zinc workpiece.
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What is your "zinc workpiece"? pure zinc or a die-cast alloy? The blackener needs a high percentage of zinc content.
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It was previously phosphated, and I have stripped it back to the metal using Birchwood Casey Blue and Rust Remover (Sulphuric and Phosphoric acid based), and steel wool.
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I'm not familiar w/the Birchwood products, but if it's got phosphoric acid, you may be re-phosphating the part, preventing the blackener from reacting with the zinc.
Why not use just 5% sulphuric as recommended in the manual?
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Can anyone recommend a better way of pre-treating the metal to get a better finish please?
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I think you need to better identify the metal alloy. If it doesn't have high zinc content, it just won't work as expected.
Sean