Caution about using liquid starch on your quilt. If you don't plan to wash it out of your quilt when you finish, keep in mind that the starch will tend to attract bugs when your quilt is stored (particularly silverfish).
Try using a product called "Mary Ellen's Best Press". The JoAnn's here sells it. Works very well and has no white residue on your fabric.
When I used to have to starch and iron shirts for work, I used spray starch that would cause the white residue problem you're having. Sometimes if you spray lightly and then give it a minute or two to sink in to the fabric that will help. Also spraying from one side of the fabric and then ironing from the other side helps, but that is unwieldy for a full quilt. Would help though if you were starching before assembling the blocks together.