Griff is right but if you already have the model, you'll need to remove the surfaces by hand. Either select them in the 3d view. If they are totally enclosed - from the side, select all the vertices that will be in one of the inside surfaces, then switch to surface-mode - the inside surface should then be selected and you can press delete to remove it. (or you can use Surface->remove-surface only, if you stay in vertex-mode)
The light blue lines you have are bad polygons i.e. polygons with <3 vertices. Use Edit->Select-surfaces->invalid-poly and then press delete to remove them. Optimizing the surfaces will also remove these.
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