I have been having problems with the colorfastness of superwash merino especially with reds. I handpaint with Cushings dye, set with vinegar and steam. Any suggestions? or fixative additives?
I would add vinegar to both the water you soak the fiber in (you are soaking it, aren't you?) AND in the dye solution. Merino can be very dense, and the post-dye application of vinegar may not be penetrating the fiber. Also, make sure that you are "cooking" the dyed wool for at least 40 minutes.