Spray starch and Spray basting
 

I am planning to spray baste my quilt sandwich but since the backing is flannel, I want to use a product that is both spray starch and sizing combined to make the machine quilting easier. My question is whether to spray starch the backing first or wait until the sandwich is basted and then starch the backing? Thank you in advance - I know you people are more than helpful!

I don't believe there's such a thing as a spray starch and sizing combined, usually people will use one or the other. I personally prefer starch. I spray starch each fabric that I'll be working on at least 2 hours before using them, then I either air dry or stick it in the drier for a few minutes. I starch the backing before I lay out my quilt to spray baste. Others may do it differently.

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.