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I would encourage a wire armature & styrene, and then build up putty from there. I think solely greenstuff, as is, will only bring you heartache & pain.using your phrase as a search, I came up with quite a few hits:
For sword blades, I use a hair grip or piece of flat wire, ground to shape with a cutting disk.
Tom Mason has a lot of sculpting videos, with at least one on sculpting swords -
From what I can remember bladed weapons are usually sculpted with a harder putty. They hold their shape better and can be sanded or planed down where straight green stuff can be difficult. Here are some links for blades using milliput. http://www.coolminiornot.com/forums/showthread.php?6273