When I am stressed out or frustrated with a quilt, I take a break and go piece a quilt. I tried the spray starch on those tough fold marks in the fabric, left it set for a minute or two to soak in, and then pressed. Beautiful pressing without any fold marks and no flaking as you will get with many starches. (Now a word of warning - if you are working on a quilt that may never be washed and maybe stored, starch can draw small bugs that can damage your quilt. It is a small risk but one you need to be made aware.)
Have a great day!
Yours in quilting,