I tested them on a tiny drawstring bag I found in the closet. The colors didn't bleed on the muslin (just make sure the light colors are completely dry before drawing the black on top). Cute, right? You could cover a whole piece of fabric with tiny designs.
The markers did bleed a bit on the 7 oz. cotton canvas I used for this zipper-zipper pouch. I learned from testing on some scraps that colored areas will likely look scribbly, so on this pouch I went with it. (Click to see it bigger.) If you're looking for perfectly even coverage, this probably isn't your product. Colors can be layered, which will get you a darker, more vibrant look, but your drawing will still appear a bit sketchy. Good for free spirits. Bad for repressed, control-freak designers who like crisp, graphic shapes (um... me). But fun to test! Full disclosure: I was too lazy to illustrate my own zipper, so I Googled "zipper diagrams" and copied one.
Something that's not revealed on the package is that the yellow, orange and pink markers are neon like highlighters—I wish my photos captured that better. The other colors are normal. I haven't tried washing anything yet, but the package states that ink resists fading on most fabrics during normal wash cycles.
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