I'm not sure JB Weld is what Nelson wants to use on a vertical joint on the backdrop, in my experience it tends to run or slump. I would go with one of the two part polyester auto body fillers; Bondo is one name but there are others. Two part fillers have minimal shrinkage, whereas the single component "spot putty" will shrink as the solvent evaporates.  The two part fillers may be a bit tough to sand when used on styrene, since they are as hard or harder. In which case, use the two part filler to fill the depression, and the knife a thin coat of the spot putty (also known as glazing putty) over the top and sand that smooth.

Dennis Storzek