I've already built a few simple projects, but I find that the ignorance
lingers, even as regards some very basic concepts. I learned a ways back
(the hard way) that "modern" ply can't be sanded a great deal without
the substrate peeking through. My error stems from '70 era high-school
shop training that was better suited to solid wood, and the memory of
older plywood whose top veneer was more than a few molecules thick.

So my question is this: How should I sand?

I'm considering building some plywood bookcases; tall (7'?) narrow (24"
x 15"?) tower-shaped things to flank a wide window. Birch, or maybe oak.
I've been told here that I need not start with coarse paper on ply, but
I'd like some details.

I'd be using a very old third-sheet orbital sander. It's what I've got,
and it seems to work well. The weight seems to damp some of the
vibration too.

http://www.flickr.com/photos/gdguari...4261/lightbox/

So what grits should I use? And how long should I sand each area? This
project, should it actually become reality, would require sanding the
better part of three sheets of ply on both sides, not including whatever
I use for the backs. I'd like to be efficient.

As an aside, what should I use for the backs, anyway? I'm sure 3/4" is
not necessary, but what do you recommend?

As always, thanks in advance.