That sounds like it's worth a try. So I wonder if a citrus-based cleaner like Oxygen Orange would have a similar or even better effect. Not just spraying the caps with it but soaking them in it. My caps are not pre-1930's luckily. Thanks for your advice.Old bottle caps are pretty similar to old beer cans, so the same cleaning methods apply. Easiest is to soak them in citric acid (or just lemon juice). Oxalic acid would likely get better results, though I wouldn't necessarily refer to oxalic acid as "safe." I should note that I'm not certain that this applies to the really old pre-1930s bottle caps, I haven't had any experience with those so not sure if the paint is different or not.