the basting spray may still work, but may not adhere as well if you starch your fabrics first.the sprays always work best when used on pre-washed fabrics with sizing, oils, chemicals removed. usually if you starch, fusable interfacings do not work. so ... if you plan to use basting spray, why do you want to starch too? just curious...i would just spritz with water if my fabric needed a good pressing before putting the sandiwich together with the basting spray and would not add anything to the fabrics (sizing or bleach) pre-washing is to remove these and to shrink/set colors...if you have a little extra you could put together a small sandwich with starched fabric and basting spray and see how well it works. it is my understanding there are big differences in basting sprays also, not all are created equal.