I have painted on fabric for years and there are many products that will work for you just like the previous posters have suggested. One thing that helps the paint to adhere to the fabric is to prewash it...just as you have already done. HOWEVER, it is important that you do NOT use fabric softener. It leaves a coating on the fabric that can stop the paint from adhering as well to the fabric. I also heat set mine...either putting in a warm dryer, or pressing with a dry iron through a pressing cloth. Just be sure to do a test pressing ahead of time. Good luck!