flip polygon? I hope he was being sarcastic! We really sould get a sarc tag....
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ummm, flip the way the poly faces. i don't wanna know what you thought

Blender is good and is free, but It allways make you model with a so called Inside of Poligons (still don't kwon what are those but ppl that have seen or sue my models allways tell me that)
it does use double sided polys if thats what you mean. if you set truespace to import dxfs in certain ways, and make sure the object is solid (ie no holes) then ts will automaticly set all the
polygon normals 
to face outwards, fixing it. (i think

)