I think you can use mild steel if you want, but I wouldn't expect it to last long against the big spinners of today. Then again, not much does. It's fine for internal structures like bulkheads, but honestly I think you're best going for Hardox on the outside, unless you plan on doing a Hypershock and just avoiding hits on your armour.

Have a look at this by the PP3D guys: http://pp3d.weebly.com/getting-start...-and-materials