Kevin,
I generally use six inch sanding bars for most work. I only use 80 grit for removal of alot of material and shaping of wood. For general sanding to obtain smooth surface, I start with 150, then 220, then 320. I have a couple 12" sanding bars and one really long one (24"?) that are good for getting matching height on wing ribs or a long turtle deck where you don't want any dips or waves. For curved surfaces (like the rounded surface of wings) find a flexible pad for your sandpaper or just use a folded piece of sandpaper in your hand to avoid getting flat spots from a rigid sanding bar. Hope this helps.[sm=thumbup.gif][sm=thumbup.gif]

For knocking down the edges on your spars, I would suggest using the short bar with some 80 grit on it, as they are likely hard spruce wood, and the short bar is easier to use for that. When you get the surfaces the same height, then switch to the 150 grit to smooth it out. A tip you can also use is to cover the soft balsa near the high areas with masking tape to protect it. It's easy to accidentally sand into an area you don't want to.