google sketchup 3ds invisible faces

when i import a model from google sketchup into unity as a 3ds some of the faces are only visible from one side, same with obj, this is really annoying because i just spent 3 hours making an M9 and now half of it is see through >:( anyone know how to fix it?

dont worry i fixed it, i found a massive mistake in the free version, i saved as a kmz then extracted the collada with winzip lol