As I was reading through this post last night, I panicked when I read that you should not use white on white fabrics. After checking with DDIL, the pigma pens worked fine on mine. I had washed the WOW before using, as the rest of the quilt is reds and blacks (which HAD to be washed before using in a quilt for a teenager). I don't usually pre-wash my fabrics. So I'm thinking that testing is the key.
Linda in MO