Glue sticks are expensive and do not go very far.....I would be more inclined to go with liquid school glue and a thin cotton batting (I personally use Warm and Natural)....If your customer wants a smooth look I would be more inclined to really pre-wash all the fabric, so nothing shrinks....perhaps also your batting....I have used Elmers school glue on my last 5 quilts with no problems at all a thin drizzle works really well and drys pretty quick, I would be more inclined to recommend you put a sheet over the quilt and allow to air dry (doesn't really take that long) rather than use the iron on a whole quilt. IMHO
