The Outer Shell tool () removes geometry inside overlapping groups or components, leaving only the outer faces.
In SketchUp for iPad, the Outer Shell tool does not have any optional tool modes.
iPad Input Methods for the Outer Shell Tool
The Outer Shell tool can be used with any of SketchUp for iPad's input methods in either Just Draw or Click-Move-Click mode.
Click-Move-Click Mode or Just Draw Mode
Mouse or Trackpad - Select the Outer Shell tool and click your first solid, then click your second solid.
Pencil or Multitouch - Select the Outer Shell tool and tap your first solid, then tap your second solid.
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For more information on how to use the Outer Shell tool in your models, see Creating an outer shell in Modeling Complex 3D Shapes with the Solid Tools.