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Quilters often wash their fabrics before cutting and sewing. It is even more important to do this if you are making a garment that will be washed. There is no quarantee that a cotton fabric will not shrink, so before spending the time and effort making a garment from cotton (or any other washable fabric), preshrink your fabric!
I have in a past note introduced an interfacing from Staple Sewing Aids - a cotton fusible. I have since found this to be a much more useful interfacing for reinforcing garment facings than for stabilizing neckties, especially the heavier silk. Though the HTC "Touch O' Gold II is still my favorite, it is rather expensive. Fairfield's "Stabilize" is equally effective for most uses and is much more affordable.