Snoopy,
I agree with Madda about Tamiya's putty. I have found it to be some of the best around. If I'm not mistaken, the Tamiya putty actually contains fine metal particles (hence its metallic coloration) that gives it some substance when it's dry, and reduces its potential for shrinkage. It's also quite fine (as opposed to coarse). I haven't tried their polyester stuff, yet, so I don't really know how it performs.
One of the things I have found that works well on right angles like you are describing is to use an automotive putty like Dupont's lacquer putty (3M also makes some pretty good putty, too. I think it's called Blue Acryl??) Basically, this stuff is thickened primer, but much thinner than putty. Because it's a thickened primer, it essentially has no grain, so it's incredibly smooth. I've only seen minimal shrinkage when dry. One trick I've learned is to lay the lacquer putty into the joint where you want it (I like to use a toothpick), then take a cotton swab wet with acetone (primary ingredient in fingernail polish remover) and swipe the joint. You'll pull out all the excess putty, and be left with a nice smooth joint. No sanding required. This process works better with angle joints, than it does with butt joints. Because the stuff is a rust red in color, you'll probably want to prime a little before laying down your base coat. I bought a tube about 12 years ago for about $18 from a paint store, and I've still not used 1/2 of it yet.
Another trick is to do the same thing as above, only use white glue (Elmer's), and swipe it with a swab moistened in water. I have found that Elmer's works best in finer/smaller joints; lacquer putty works best in slightly larger ones, but is not good in large crevices (use super glue or Tamiya putty).
With most putties, you can spray your base coat right over the top; however, I like to prime the surface occasionally for the purpose of locating any shrinkage, or other areas where the putty didn't fill completely, or where additional sanding is needed.
Hope this helps you some!
Gip Winecoff