I use the Frixxion pen all the time and love it! I have the black one and while it is great for lighter fabrics, it might leave a lighter colored line after ironing off the black.....it's no issue for white or lights, but for darks it can be visible....I've heard that it will wash out, but haven't washed anything I've marked yet. They come in many colors too. I am going to try the washable crayola markers next....I hear tell they are better. But with whichever way you try to mark your quilt, do yourself a favor and try a sample sandwich....mark it, then wash it to make sure it comes out.