YOu need a stabilizer. Are you thread painting as part of the quilting, or before layering the quilt? I'm not an expert at thread painting, but have done a little. I like to use Ricky's Stable Stuff as a stabilizer because it softens with washing and just adds a little more "batting" to that area. If you're doing it as part of the quilting, the batt and back should act as stabilizer.
I prefer to thread paint before quilting, because quilting mashes the fabric down where it is quilted, and I want my thread painting to pop out, not be mashed down.
Hope this helps.