Eric,
If you're going to use the sonotube, you're probably going to use end rings to re-inforce the tube, so here's a trick. Slide the ring onto the tube and use it as a cutting guide for your jigsaw. This gives a smooth, square cut. You will have to mount the ring to the tube so that it doesn't move while you're cutting; I use drywall screws and a power screwdriver. It's better to put the guide on the part of the tube you're cutting off, otherwise you'll have extra holes in the tube from the temporary mounting screws when you move it out to the end.
As for finishing it, peel off the inner layer of "wax paper" and finish it with one coat of flat black. (I say one because my second coat of flat black left it glossier than one coat did). As for the outside, either strip the waterproofing with solvents, or peel of the outer layer and paint. Peeling it doesn't look as good when it's finished though.
Hope this helps.
Bob Bob Lombardi WB4EHS in Melbourne, FL (>))))||||| blombard@iu.net Microsoft just notified me that they are filing suit against me for using the name "Bob". You'll have to call me by my new name "Windows '95".