I prewash all smaller pieces, but I do it the old fashioned way. In a tub ( or large bowl), using dawn dish detergent. I fill up the tub with warm water, add a tiny bit of dawn, add the pieces of the same color and swish. I leave it in the tub over night to let the dye ooze out. then i drain and rinse, and add a 1/4 cup of kosher salt, let it sit for a bit, then rinse and hang dry over the shower curtain rod. this is the way we did tye dye back in the 70's, but we heated the water/salt to set the colors first.