Over the last couple years Depron has been coming in with a slight cup to it. The 3mm and 6mm sheets are workable, but lately the 9mm has more cupping than some customers like. The manufacture pretty much says “Take it or leave it”, being that it comes from Germany and there is no other source in the USA, I’m stuck with what I get. Below are a few steps that will allow you to get it flat again.
To flatten the foam we suggest a paint roller, a pie crust roller or a piece of large PVC or cardboard tube. Take a couple large towels and fold them in half to make a nice cushion. Lay the parts out on the towel, with the high point of the warped foam facing up and roll the foam with moderate pressure Take your time and don’t try to do it in one pass. It is best to do this technique after you have taped your hinges and before you assemble the kit, if it’s completed you can have someone help by holding the wing while rolling the foam. Just be careful around the areas with sharp corners like counterbalances and also with cutouts like servo mounts. You can still work over them but keep off the heavy pressure while rolling over these areas.
Keep in mind that with the tabbed design of construction, the fuselage straightens out quite well when assembled and the wing spar and the taped hinge line removes most of the warped foam.