I have used both styrene and 1/8th inch masonite for backdrops on my current layout. Masonite is stronger and self supporting when coved. Styrene is easier to work with but requires supports to keep it in a coved shape. In both cases you need to prime. I used water based Kilz for the most recent backdrops, but decided that regular latex primer or gesso is better. I used those earlier.
For fascia I used both 1/4 and 1/8 inch masonite. 1/4 inch is easier to countersink screws. But lately I've been using 1/8th inch and number 6 screws with the 1/8th masonite. I spackle over the screws and paint with textured paint. Makes a good solid layout fascia. Nothing makes a layout look better than a backdop and fascia. Add some terrain with a basic earth color and many people think the layout is done.
Ths llink will take you to my blog posts about backdrops if you care to read more about my experiences with the various materials.
http://usmrr.blogspot.com/search/label/Backdrops