A few weeks ago I dyed a dress that was faded with RIT dye. RIT claimed that the dye was permanent, and it might be, but it certainly wasn't colourfast. I looked everywhere on the packet and on RIT's website but could not find any information to say that a fixative was needed. Mary from Biblioblog kindly left me a comment to say I must use a fixative.
The next day I was on the internet finding out all about fixatives. Many products cannot be imported into Australia, but this product was available for delivery to Australia via fishpond.
The product is called Retayne and it is for use with cotton fabrics only. It prevents colour bleed in dyed fabrics, whether from your own dying or from commercially purchased cottons. It seems that it is used by quilters for treating patchwork cottons as well.
You use 1 teaspoon per yard of fabric, hot water, and soak for 20 minutes. So I fixed my dress and another garment as well. The second garment bled on being immersed but soon stopped.
You are advised to wash treated garments in cool/cold water after fixing, so one assumes there is still a tiny bit of colour run potential left. I understand that after treating with this product, the treated fabric will be more light sensitive, so it is best to then dry them out of the sun and light.
I hope this helps anyone else that wants to dye fabrics, or has fabrics that bleed, or wants to do patchwork.
Thanks Mary for your helpful comment :)