I have one of those blue water-soluble fabric marking pens, and generally I never get around to actually removing the marks (more typically I forget about them and then accidentally heat-set them with the iron ::)). Well, this afternoon I started working on a corded bodice UFO and decided there was no compelling reasons to keep the blue lines marking the finished boning channels and grommets... It wasn't until I was spritzing down the first panel, watching the blue disappear instantly and completely, that it occurred to me that I made those marks A YEAR AGO. Wow! Never would have expected it to work exactly as advertised after all this time.
It's probably not something you'd want to *rely* on, but for those of us whose projects are reeeeeelly long-term, it's nice to pull something out of the stash and have all your marks still intact if you need them!
Wow, what brand is it? I need to find some of those. I have had bad luck with the marking pens, pencils, and markers I keep buying.
See, I prefer the air-soluble ones, because I tend to not take the time to wash them off. Occasionally, if I have something that takes me over a week, it can be annoying (the marks are usually gone by then). My favorites are the purple ones (white with a purple cap) from Joanns, since the pink ones never seem dark enough.
My favorite's the air-soluble one, too, just because I really like how well it marks, although the one I have right now is unpredictable in terms of how long it actually lasts. It seems to depend a lot on the weather and the fabric. It only lasted about twenty minutes on the fabric I used on my purple dress, which presented a challenge when marking the skirt for cartridge pleats. ::)
Lady Shadow, I'm not sure what the brand was; I have a couple of them. One marks better, but doesn't remove as easily. And one makes a finer line (which I like) but sometimes doesn't show up as well without multiple passes.
I think my quest for The Perfect Fabric Marking Tool will forever be unfulfilled, although I keep trying! For some applications, I really like my Chaco-Liner (http://www.elegantstitches.com/Notions/ChacoLiner.jpg), though.