The best way to determine exactly what is causing your problem, is to to have just the one object in the scene (with a black, empty background), and then look at it from the inside at various angles. If you can still see through it at certain angles, then it is probably a problem with the normals. If you are instead seeing odd triangles showing up that are not black, then it is probably due to some stray polygons in your model that shouldn't be there, which are protruding through the model, and only visible from certain angles. If everything looks fine from whatever angle, then it is probably a problem with parallel faces being too close to each other.