I just plied up some cotton/wool yarn and now I'm wondering how to finish it. Cotton is boiled to finish, wool should never be boiled. Also, when making a two-ply with one ply of cotton and one ply of flax would you finish these in the singles, before plying? I finished a cotton plied with flax by boiling, and the cotton shrunk to half it's size while the flax didn't. It is very usable, but I was wondering how others finish when plying different fibers.
In recent years, fewer spinners are boiling their cottons. I teach and have experimented with several ways of handling cotton. If you want to darken the cotton yarns, Pima can turn creamy, browns and greens can get darker, then simmering is required.
However, you can just wash your cotton yarns like any other yarn, in the sink with some detergent. I always wash my cotton and other fine skeins, as twisted skeins. This prevents the twist from shifting too far in the short fibers and drifting apart and from the fine yarns tangling on each other. It will be helpful to run a short yarn through the two loops to keep them from slipping apart. A twisted skein is very easy to wash and if it is cotton, even dry in the dryer. Your next question of finishing blended yarns, depends on the ingredients. When I teach blending I always recommend finishing the yarn with the requirements of the most fragile fiber. However, just as other teachers recommended the "abuse" method - slapping a wet skein around to encourage felting, which in turn felts the wool around all other fibers, every rule or suggestion needs to be considered by the results desired. If the two fibers are in separate plys, then I call it a mixed yarn, and great novelty results can be obtained by the finishing or abuse you give the yarns, since they can independently shrink, or full, but will pull the other ply along. Try a bit and see.