I have done up to 2", thick wall diameter, can't remember what size though. The trick is to use muliple heat guns and get it heated evenly. You want is hot enough to be plyable but not too hot that is starts caving in and kinking. Take it slow and work with it. I know some people pack the PVC with sand to keep it from kinking, never done it myself but they say it works. I have done the packing thing when working with steel and it helps, so could be worth a try.
Wrap the pipe with flocking or thin soft foam then cloth. that will smooth the texture out and give you a nice painting surface.I considered doing something like that years ago (but the need for it went away.) I was going to use plywood to make a form and use dryer hose rather than drainage pipe. I never resolved how to smooth it out, though I had considered using red and white duct tape to give it a smoother finish.