To pre-wash or not to pre-wash, that is the question. We get calls from experienced dyers as well as "newbies" asking why their dye job came out uneven. One of the most common causes is skipping the pre-wash. In our directions, we recommend pre-washing garments with HOT water and a detergent like Synthrapol. Some dyers use soda ash in the prewash. This is called "scouring". This process prepares the fibers to receive the dye by cleaning them thoroughly. Even PFD (prepared for dyeing) garments can come out blotchy from not pre-washing. That is because fabrics and garments go through a lot of machines and hands (including yours!) before they get dyed. Any oily substance (seen or unseen) can inhibit dye from absorbing. Fabrics and clothing not specifically sold for the purpose of dyeing can have other substances on them which inhibit the dye. Pre-washing as recommended above will remove just about everything except starches and permanent press finishes. An enzyme detergent will remove starch. Permanent press garments are best not dyed unless you want to mess around with Muriatic acid (available at pool supply stores).
Dharma sells fabrics and garments which are intended for dyeing, and we specifically ask our vendors that they not be treated with any "finishes", so hot water and Synthrapol detergent should do the trick. For the reasons mentioned above, pre-washing is most important for tub dyeing when you want to get an even solid color. Folks doing tie dye, batik, crystal wash, low immersion or other "un-even" techniques often choose not to pre-wash for reasons of time and economy. If you get satisfactory results, great! But for solid color tub dyeing we don't recommend skipping this vital step. If you don't have time, you will sometimes get lucky, sometimes not. You have to decide if you can accept the results if they come out uneven.