Cold-Rolled Mild Steel is the standard for beginners, and remains a dependable default material in this craft even for experts. It is generally cheaper than other metals, and its malleability is excellent, yet yields a very hard armor product once hammer-hardened. It is heavy though, and its worse characteristic is rust. It rusts so fast that…well it’s rusting right now while you read this. It rusts more if you look at it. It rusts faster if you touch it. Breath on it and the rust gets turbo-charged. So if it gets rained on…oh good heavens you have a mess. There are ways of dealing with the rust problem though. Coating the metal with a fine sheen of petroleum jelly is one great way to nearly stop rust on mild steel armor. Some guys use an automobile wax product, and some have found some expensive unique rust-blocking sprays.
Storing armor is also important for minimizing rust. Do not stack your armor in pieces together and shove them under the bed or onto a closet shelf or floor. The pieces need to be stored in such a way that, even if coated with wax or oil, they are not in contact with anything else, even other armor pieces or surfaces. Ideally, this means using a display stand. You’d rather display your armor anyway, right?
Don’t be afraid of rust though. Dive head-first into this craft with cold-rolled mild-steel (CRMS) and deal with rust like the rest of us.
Even expert armorers who typically make armor out of other metals, will often go back to mild steel for developing new things. It’s cheaper to make mistakes and fine-tune a new custom pattern with CRMS than go straight to stainless steal. So any medieval plate armor shop will typically have some supply of CRMS even where most armor items are made from stainless.
The standard thickness for medieval plate armor made from CRMS is 16-gauge, but 14-gauge for helmets. Some fighters prefer their entire suit be made from 14g and their helms made from 12g. Sometimes a set of 16g legs will have 14g knees for extra protection at such a critical place. Thicknesses vary depending on the craftsman and the wearer’s preference, but do not get thinner than 16g (CRMS) for real armor to be worn in combat. Anything thinner than 16g (CRMS), is considered to be costume armor and should not be used for fighting. Thickness does however vary with other metals like stainless steel.
Buying this sheet metal is as easy as finding a sheet metal supplier and telling him you need “a sheet of 16-gauge cold-rolled mild steel”. It is common stuff. He’ll ask you if you want it cut. It is advised to say yes, depending on their shearing fee, and if you drive a Honda. If you say, “I only need it cut in half” or “I only need it quartered”, they might do that for free. He might ask you what it will be used for. (Guys who sell sheet metal are always interested to know what their buyers are making.) So tell them! They’ll likely say, “Oh yeah, you’re one of those guys.”