Everyone knows you should dye with clean fabric. But if your dye still streaks after you’ve run it through the wash, then your laundry detergent probably isn’t doing the trick. ‘Clean’ fabric is free from chemicals that interfere with the dye process, like sizing and preservatives. But there are interfering chemicals in laundry detergent too—chances are the Tide sitting on top of your washing machine at home has color-protecting additives and ‘Mountain Breeze’ or ‘Cuddly Lavender’ scent (okay, I’ve never seen ‘Cuddly Lavender’.)
Try using Synthrapol, a scouring agent that also comes in handy when you’re rinsing your fabric. Not only does it clean your fabric before you begin to dye, but added to the rinse water after dyeing, it also removes excess dye from your fabric that will end up on your skin or on that nice white fabric you have on your costume.
Sometimes, fabric will need more than one scouring wash with Synthrapol. To check that it’s clean, use the water-droplet test. For most fabrics, if a drop of water sits on top of the fabric instead of getting absorbed into the fibers, your fabric still has some residue.