Thank you for all your replies, very useful!
sewgull:
Re stiff brushes - do you tape your fabric down when you paint it? I painted a line on fabric the other day and tried a stiffer brush than normal, but it seemed to distort the fabric too much. Maybe just a matter of practice.
dotcomdtcm:
Aren't Procion dyes transparent? There are a couple of darker/patterned fabrics that I'd like to paint on and I was under the belief that thickened dyes would disappear on colours that are darker or like in value. If I'm mistaken, great! I've wanted to try Procions for ages.