After you roll it it will have little wrinkles in the skin.
Us a heat gun from a distance and wave it back and forth and work your way closer until you see the wrinkles start to get smooth.
You will see what happens when it gets too hot when you practice.
The trick is to know when to stop - almost always you need to stop before it gets perfect.
I find it hard to stop
Another side effect is that it may take some of the bend you wanted, out
You can harden flat pieces, too.