I use the spray (505) and some curved pins. I do it on the kitchen table which I cover with broken down cardboard boxes taped together. I can't stand to do it on the floor (kills my back)..so first I lay down the back..square it to my marks...then I tightly clip that sucker to the table with those big notebook clips from the Dollar Store. Then I lay the batting on top..square...then I peel back a section and spray it lightly and smooth it back down..I work my way around the table doing this...then I do the top the exact same way. I then flip it over and smooth any wrinkles that appeared, then flip it back and pin any "critical" parts. It goes pretty fast and holds together extremely well. Of course, I do grumble the entire time.......
I use the washaway thread when I'm doing trapunto and it does wash away really well when you soak it in water...lightly swishing it around...I don't know how it would work for basting. I think it might be pretty expensive, but might be worth it?? :roll: