these are the primary tutorials I use for my own works, I have not include NMM tutorials just basic metals.
Sukigod's crafting battle damage, great tutorial with excellent pictorial reference:Battle damage
Ultawerke' rust weathering tutorial using MIG products. MIG specific as I use the same product but I am pretty sure the same principles will apply to other brands:Rust/Weathering Tutorial
Hope these help with your project :)