making sharp inside corners

Thanks for the replies! Those are good ideas and I will give them all a try.

Tom;

 I took a square chisel blade from X-Acto and did this . I made a notch in it with a brand new grinder wheel .I then squared the inside corner with a file .I didn't make it very deep because of some of the places I would use it  .

     The whole reason is what you are facing . Most planes don't have a square corner at the top or bottom of a wing locator point or fillet .  So you scrape to get the face clean but not cut a groove in the slightly slanted surfaces of the fuselage .

 This allows you to create those hard but necessary square surfaces for joining a wing such as those of the X -3 . Take the wing and clip off the mounting plate .Put a plug in the opening for it .

   Then do this . Drill a hole in the new area , one in front toward the leading edge and then another around the forward edge of the flap . Insert the pins coated with glue in these holes . Now you drill the matching holes in the wing . Glue the holes . Slip the wing  on and let dry . T.B.

Scraping is another way to get a sharp corner.  Scraping is dragging a sharp edge along a surface with the blade somewhere near 90 degrees to surface.  You need a blade with a 90 degree corner in its profile.  Not very common, but if you have an old blade you do not want, you can modify it with a Dremel and grinding wheel to get that shape.

What works for most of my sanding including sharp corners is to make sanding "blocks" from thin styrene stock, .010" - 020" depending on how flexible you like. Most of the time I'll use a 1" x 2" size, which fits my hand, sometimes bigger or smaller for specific tasks.

I like to make them in bulk, cutting 1" x 3" strips of 400, 600, 1200 wet & dry sandpaper, then glueing them to the styrene with thin rubber cement. Any craft cement that bonds parper to plastic works as well, like Ailene's Tacky Glue for example.

These little sanding pads can be used wet or dry, and conform to curved surfaces. The folded over ends are really handy for tight spots.

Mike