I have a 3D gasser in need of major wing repair. Because of the severe taper and low rib count, the sandwich method did not work very well and so I used the following method.
1. Make templates and cut a foam wing core.
2. Leave on the 'shucks', mark and cut at the rib positions
3. Use the ends of the foam segments as patterns to cut out the individual wing ribs
4. The foam segments can also be used as jigs when rebuilding