just went through this myself. flat paint can do a lot to hide those seams and battens, but the best solution i found was textured wallpaper. it looks like textured drywall when installed and is very very forgiving of wall irregularities. its also much easier to work with than other wallpaper as there is no real pattern to worry about and the seams dissapear when you paint over it even if they were a bit uneven. in one room i also used roll on texture to create a very heavy textured look and that worked ok but was much more work. if you are just doing a wall or two consider drywalling over the old stuff. primer followed by knocked down texture will also help and is pretty easy, but it won't hide the seams completely. all the big home stores carry the wallpaper and its fairly inexpensive. you may have to prime first, just do a test area and see how it sticks.