The pens are the same for sbing & quilting. I used this pen on a quilt I made 10 years ago and it stills looks as good as the day I wrote it. I do not prewash fabric. I personally think the problem was not letting the pen set in the fabric long enough. As far as I know heating it has no effect to either set it or not set it, I think maybe you should leave it on the fabric overnight and see if you have better success.
Cheers!
Rachel