Synthrapol is a soap that suspends unset dye particles in the wash water so they are rinsed away rather than settling on other fabrics (bleeding). I don't prewash most of my quilting fabrics, so I like to use Synthrapol for the first couple of washes for my quilts. Synthrapol makes sure that if there are any fabrics that bleed a little, the dyes won't stain other fabrics. Synthrapol is a gentle soap too, so I prefer using it over laundry soap for washing everyday quilts.
Synthrapol can also be used on quilts that have had a fabric bleed. Often the bleed isn't completely set, even if the quilt has been dried, so washing several times with Synthrapol will often release the bleed.
Retayne is used to set dye in fabric. I would *never* use it to wash a quilt! However, I do use it if I have a fabric that bleeds a lot to set the dye. Although I don't prewash most of my fabrics, I will often test a fabric (especially reds, purples and blacks) because a big bleeder isn't worth using in a quilt!
Many quilt shops carry bottles of Synthrapol and Retayne now, especially quilt shops that cater to dyers. They are fairly inexpensive. I think I paid $8 for a large bottle of Synthrapold last month.