Hi YHWH_777,

You can't remove a fillet where you end up with the original shape before you applied the fillet, you have to delete the fillet and rebuild the surfaces that the fillet trimmed away, this is called low level repair, Michael has a mini tutorial on this at the following link; http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

You'll have to way up if it's worth doing depending on how much detail you have done on your model, sometimes it would be quicker to model it again, it all depends.
It's good to know how to do low level stuff, the more you do the more you understand it and the faster you get.

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