I'm more than happy to give you the relevant information from the tutorials I've read, as one who is interested in the miniatures creation process. I admit I'm not an expert, and don't have any practical experience. All I know is that the choice of material could help you with the sculpting of flat surfaces. You might want to try Milliput, possibly mixed with green.
I think you can still find the PDFs of the tutorials mentioned still somewhere on the internet, but here are the relevant parts from them.
This excerpt is from Sculpting Basics, credit goes to Kevin Chenevert of LyonStudio.com.
"Secondary Sculpting Medium - Kneadatite brown expoxy ribbon. Used alone or mixed with green, it is used for sculpting sharper edges and machined looking parts. Sands well. Cures more brittle. Optionally, fine white Milliput can be used or mixed with green."
This excerpt is credited to Bob Lippman, called Sculpt Your Own figure (used to be in Total Power issue # 1 Warhammer Fanzine).
"While technically an epoxy putty, Milliput has an entirely different consistency from Kneadatite. Where Kneadatite is elastic, Milliput is clay-like. Where Kneadatite has a uniform consistency, by adding water to Milliput it can be either thick as clay or as thin as milk. Where Kneadatite is flexible, Milliput can be brittle. Finally, while Kneadatite can be cut with a knife, Milliput dries like stone and can be filed, sanded and drilled. there are times when nothing else will do. A good example is when you are trying to make something with inorganic angles and flat surfaces, like a spear tip."
Apparently you can also do it with Kneadatite, but green isn't really recommended for inorganic flat surfaces. However, Titanium Kneadatite apparently works.
Can't really find who wrote the article, but there's a Chronopia example by Excelsior Entertainment showing sculptor Matt Beauchamp doing a step by step mini of a Shadow Stalker.
"Taking a straight wire and bulking it up with Titanium Kneadatite, you will be able to get a jump start on the sword as well. Why a different kneadatite? Basically, the Titanium sets stronger and more rigidly and when filed (to create edges), it retains the shaping and holds the edge better than green."
From the looks of things, using Milliput (possibly mixed with green) and sanding the surfaces is the best way to get flat, angular surfaces. The green mixed in is I assume is to help retain the shape when the sculpt under goes the vulcanization process when making moulds, as green stuff has good heat and pressure properties. However, it's probably recommended that you still reinforce things like flat edged surfaces like swords and spears and surfaces with wire underneath. But then, like I said, I'm not a sculptor, so you might want to wait until an expert wades in and gives you the low-down on how it's done.
If I may ask, what are you trying to create? Seems to be some sort of robot or powered armour thing for a science fiction mini?