The directions for both say to use HOT water (140 degrees F., 60 degrees C)
The instructions for Retayne suggest treating the fabric BEFORE cutting it.
Retayne - soak fabrics for 20 minutes in hot water, then rinse
Synthrapol - soak 10-12 minutes in hot water, then rinse.
As far as I can tell from that info, they are NOT interchangeable.