I found this it may be helpful
Retayne actually SETS errant dyes so they'll stop bleeding, the Synthropal
"floats" out any unfixed dye and keeps it from "fixing" itself onto areas
of the fabric that you don't want to be stained. I used my entire gallon of
retayne as a "prewash" for my new fabrics, Now that it's gone, I'm using
the synthropal for the same purpose. Just from my own "bleeding thread"
experience, I'd say try it first with the Synthropal - you don't wnat to
"fix" the dye that's already moved to the wrong areas of the quilt, you
want to "float" it away. Mary Beth
Synthrapol is a "super surfactant" meaning it scours fibers of sizing, oils
and other stuff that might interfere with dyeing. You use it to prewash
fabric prior to dyeing. Its other function is to remove UNREACTED dye from
fiber after you finish dyeing something. So dyers wash freshly dyed
material in it to get rid of unreacted dye--which is the cause of most
bleeds. Synthrapol DOES NOT work to set dyes. The soda ash or salt that you
add to the dyebath has that function. What it does do is get rid of excess
dye that will, if not removed, bleed the next time the fabric is wetted. So
the Pro people were right -- it doesn't set dyes. But you still need it if
you want to use it to prewash quilt fabric to remove any unreacted dye
before you put the fabric in a quilt. It would be heaven if all fabric
manufacturers used something like Synthrapol to remove the bleeds. But they
don't. And some intentionally leave the excess dye in the fabric to make it
appear darker when you buy it. So that's why prewashing with Synthrapol is
a good idea for suspicious fabrics or even those that you can see will
bleed a little.