yes wash the sizing/chemicals out of the fabric before doing any treatment---doesn't matter if you are coloring, dyeing, using fusable webbing- anything like that the *stuff* should be washed out before using. and i've always used 100% cotton and had it hold up quite well. i just make sure to wash the finished item in cold water. when i tried a blend the color did not set at all. same as dying- the dyes do not work for me on poly blends---only cottons.