Synthrapol suspends unset dye particles in water so they don't settle into fabrics. Bleeds are not permanently set, which is why Synthrapol can work to get rid of bleeds. This is why Synthrapol works well when washing quilts. If there is a bleed, Synthrapol will keep it from settling into other fabrics.
However, Synthrapol will not permanently set unset dyes. A fabric that has unset dyes will continue to bleed after many washings. It's a manufacturing defect. Retayne will permanently set dyes, including bleeds. This is why you never want to use it on a finished quilt. If there is a bleed into other fabrics, Retayne will permanently set the bleed.