Before I learned about this trick, there were any number of projects that were ruined because of chalk or pen that says it’s easily cleaned or brushed off but never fully comes out of the fabric I’m working with. When I was working on my Desmond Miles hoodie, I had to mark the front with tailor’s chalk for all the extra seams and top stitching. That hoodie has been washed several times, I’ve had the costume piece for a couple years now, and the chalk is still clearly visible on the front of the hoodie. At a point, I’ll need to remake the hoodie to fix this problem. I’ve also seen this happen with mark-be-gone and disappearing ink pens. It’s a huge frustration that seriously mucks up a project.
Now, whenever possible, I use soap as a marking tool. You simply wait until you have a thin sliver of soap left from a bar and use it like you would a piece of chalk. When you wash your piece, you are guaranteed to have the soap come out, unlike with the other marking tools I mentioned. The only problem I ever have with this method is with light colored fabrics. If I could find a darker colored soap, I’d probably be set.