I have better luck with spray basting over pin or thread basting. I have less wrinkles on the back side...actually I rarely notice any wrinkles these days.
I've got a piece in the closet that I sprayed last year sometime, it's still together, waiting for me to return to it.
I spray lightly, in a grid pattern every 6" or so. When I get to the edges, I aim the can towards the center of the quilt to avoid overspray.
I always wash my quilts when done with them. I wash on delicate using my normal liquid soap, then dry on low.
If your fabric is stiff, you may have used too much. It hasn't happened to me, so I can't be sure, but I would think it would work just fine.