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Perhaps the Coca Cola works because it has phosphoric acid in it?
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Yep, the acid works on your teeth, too!
I agree you may be overetching, the recommended percentage on the package is probably for eating through copper, getting a pattern on steel requires less stength.
When I want to check out a pattern, I polish a small section up to 1500 grit with sandpaper and give it a short bath with very dilute FeCl or nitric, this shows up even really subtle structures that tend to get muddy or indistinct with more thourough etching.
If the pattern is fine-grained, a rough finish (120-220 grit) can entirely obscure it, 600 - 1500 is easier to get something to show.