Problems with surface in the game engine

Hi, this is my first post. Sorry for my english, i’m italian. I’ve got a problem with the Blender’s game engine. I’ve imported a model into Blender: there are lots of extructions. It renders fine, but when i pass into the game engine, mode textured, i can see through the surfaces! I can understand why it happens…
please answer me as soon as possible… thank you! BYE

It is because your normals are not all facing the outside. Many times normals get flipped in extrusions (I don’t know why that is). Go into edit mode and find the button at the far right of the edit buttons that says “Draw Normals”. Increase the value above the button until you can see a blue line sticking out from the center of each face. If you cannot see the line on some faces, but only a dot, then you know that the normal is not facing outward on that face. Try pressing [Ctrl+N] to recalculate normals outside. Sometimes this works. If you still see some faces where the normal is not pointing outward, select those faces and press [W] and select “Flip Normals” from the menu. Your object should display correctly in the Game Engine after you do that.