Starch works well with flannel, but the way I have used it is not in spray form. I mixed a 1:1 solution of StaFlo liquid laundry starch and water, "painted" the starch solution onto the backing using a large wall painting brush, waited a couple of minutes for the fibers to absorb the starch, tossed in the dryer, then ironed with steam.
Ideally you want to starch the backing first, then spray baste. Spray starching after layering isn't as effective, but it still helps a lot if you are getting puckers and tucks in the backing.