To make metal armor you have to Place metal plates on leather armor. To obtain metal plates you either have to make it or smelt metal parts of barrels or forge Them with a forging station if you have metal fragments there can be 4 types of barrels types completely Rusty, alot of rust, some rust, no rust.
To remove rust it is possible to find anti rusting spray which is 1 in 10 crates and has 50 uses then IT can be refilled with An large canister of anti rust liquid which is 1 in 25 crates, a forging station Costs 100 Stone and 25 metal fragments forging parts could be added with different forms,you could make the metal plates leather armor with Rusty armor but IT Will last less and the metal can break and then IT is a normal leather piece.