I would never spray baste when using the "birthing" method! Maybe I am not understanding something here? Spray basting will make the surfaces tacky, which in my mind is going to make the entire turning process a disastrous mess.
You also do not want basting spray on the right sides of the fabric. How on earth could you quilt surfaces that have basting spray on them? They will be tacky!
I would spray baste in the normal manner but leaving the edges free of spray (cover with paper), quilt, trim the batting to size, then trim the top and backing edges, turn them under and iron, then hand or machine sew the edges closed.
Sorry, I am just not understanding at all why anyone would recommend using spray basting with the "birthing" method of turning a quilt! Am I completely clueless here (which, by the way, is entirely possible
). To me, the key phrase of "sewing like an envelope" and then turning means the birthing method. Maybe I am off on that???