I have used Masonite or tempered Hardboard for mounting photo backdrops. You could probably use it for painting them on. The advantage is that you can have it cut to 1 ft X 8 ft strips (or what ever height you may need). Having 8 ft with NO SEAMS makes for a more convincing backdrop.
The way I mounted mine was to make the height tall enough that it went below the layout surface so I could bolt it to the layout framework with wing nuts. This allows me to remove and change the background as needed.
Also I use 1 x 6 inch filler strip so I can sandwich several "depth giving" building or scenery cut outs an inch away from the tall background. Although I only use one filler and cut out strip, it is possible to use several to increase the "depth" of your background. Just be sure you buy bolts long enough to accommodate the combined thickness.
To get additional depth you can glue litchen/foam/pillow stuffing to your painted trees.
Paul Goodness
