so i've been trying to port a model into SFM using blender but it's been a bit difficult.

i originally got it into SFM okay but i soon found that some of the faces weren't being lit when i shone a light on them, see here: http://i.imgur.com/iePR2DX.png

then i turned on backface culling in blender, found the faces that were acting up then duplicated and flipped those faces so they showed up on both sides of the model. once again it seemed great until i tried lighting it from behind and the lights started shining through some of the faces from the other side: http://imgur.com/a/BpHba

i don't really know how to fix this and google didn't show up with anything too helpful, but maybe people here can help. it's also worth noting that i'm not experienced with blender OR porting to sfm.

i'd really appreciate any help and can give more details if needed.