Hi,
I'm using polystyrene. It can be glued, and sanded very good.
If I have to build round corners, my procedure depends on the roundig I need.
If it's a small round corner, I glue 2 plates in 90 degrees, glue a small piece behind in the corner and let it dry. After that I pre-build the corner using a cutter and after that I use 240er sand paper followed by 400er and 600er paper.
If you have a look at my
Mercedes Benz New Actros thread, you can see what I mean.
If I need a long rounded area like the roof of my Mercedes, I start to form it by bending it over a broomstick or something other with a rounding that fits to my needs. In addition you can warm up the plates with a hair dryer. Maybe you want to have a look at the creation of the
mud wings of my C509.
I hope this helps you to go on.