Carbon steel and mild steel plate armor (including helmets) and plain steel mail are highly susceptible to rusting, typically being exposed to water in bad weather and sweat in good weather. Maintaining them in rust-free condition requires some dedicated effort on the part of the wearer.
Plate armor and helmets, unless cleaned by your dealer, are typically greased or oiled and plastic wrapped when you receive them. Careful treatment at this point can save hours of work later. First remove all of the grease or oil using mineral spirits and a cloth or paper towels. Make sure that the cleaned surface is thoroughly dry. Then apply a coating of wax to the entire surface of the exposed metal – use Q-tips in the nooks and crannies. Renaissance Wax™ is wonderful stuff but can be expensive on big areas of plate. Johnson’s Paste Wax also works well and is more economical. Allow the wax to dry for a couple of hours and then buff it lightly with a soft cloth. It’s OK, even beneficial, if the wax also covers leather straps and fittings.
Your plate armor is now ready to wear or display and you should make every effort to prevent rusting. Above all try to keep the armor dry but if it does get wet, dry it thoroughly as soon as possible. After each wearing, clean off any grime and re-wax, paying particular attention to areas where the coating may have rubbed off. Store the waxed armor in a dry place and check it every couple of months for signs of corrosion – if any is found, the rust should be removed by rubbing gently with a mildly abrasive pad dipped in paste wax (those green dishwashing pads work well). Pieces used for display only should also be checked every couple of months and cleaned and waxed as necessary.