If you sand ribs, fillets, gussets or anything else similar you should consider a Pilot Petes Easy Sander.
Locally made. Help me support a fellow modeler.
These are a simple and great way to sand accurately at a 90 degree angle. By angling your pieces on the board a variety of angles and curves can be accomplished.
Use on balsa, ply or anything that sands really.
If the pics are not clear enough this is basically a base of 20 cm square 16mm plywood with a machined grove that the sanding block slides smoothly up and down. Hold your material against the support for 90 degrees or on an angle to accurately sand at your chosen angle.
The sliding piece (shaped like an old duster from my school days) has different grits on each side and can be easily replaced.
Simple and effective tool every builder should own.