I had a light bulb moment once when I dripped toothpaste down the front of a black t-shirt I was wearing. Now I mix toothpaste (make sure it's white, not gel) with enough water to thin in a small sprayer and spray over the stencil. The only downside is that I have to be patient while it dries, but it leaves a nice crisp line that doesn't rub off and washes out completely even if I've ironed over it. I've thought of using rubbing alcohol for faster drying but haven't tested it. Just know that water works. It even works on other colors, even white, although it's a little harder to see.