Wing Skin Truing Tool
I made my own for less than $5 at Home Depot and stuff I had laying arround my shop. What I did was:
a) Put 150 grit on one side of my 4' level(the straight one).
b) Picked out the best 1*3 firring strip from Home Depot
c) Cut the 1*3 into a 4' and 3' piece
d) Glued/screwed(countersunk wood screws) 3' piece onto centerline of 4' piece. It will look like a giant wooden t-bar sander... Rounded off all sharp edges...especially the 3' handle part!
e) attached 4 - 4" pieces of 100 grit paper to ends and middle on bottom or 4' bar
f) clamped sandpapered level to front edge of workbench
g) taped a piece of 1/8" by 3" by 48" balsa just in front of sandpaper side of level(serves as a bearing that will not marr the finish balsa when truing edges)
h) place balsa to be trued in front of level... if the edge is way off from straight, trim with an exacto and straightedge. If it is close I just start sanding the edge by placing the t-bar on top of balsa and working it back and forth across the level until the edge is perfect for gluing wing sheets together... this only takes a few moments per side of sheeting.
Total time to make - less than 30 minutes
total time to perfectly prepare sheet edges for a wing - about 5 - 10 minutes minutes per wing depending on size...
I can't imagine anything you can buy that will do the job easier or better. Best of luck!