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Yep, vinegar works better. Tying the rusty objects into the fabric also helps. Here are the same rusty bottlecaps I used yesterday, only I used rubberbands to tightly tie them into the fabric. Then I put vinegar into a plastic bag with the fabric and let it steep for about 12 hours. Voila! The shot above shows most of the fat quarter.
And here's a detail shot:
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The stains are darker and more well defined. And the rest of the fabric is a light reddish brown color, too. Next time, I'm using vinegar instead of salt.