I always use 3 or for different types of metal paint and rub with a cloth in ONe direction, rather like a grain.
Humbrol paints are good. Gun metal, silver, darker silvers etc. Add them on lightest first to darkest. Add water to it if you get brush strokes but if you use a cloth you won't.
This is my girlfriends Zam Wessel chest armour
The 'dirt' is just old old paint purposely lefdt to go tacky. If too hard add some fresh and mix it up. Add some glue to it too. Dab the mix on with a cloth, let it dry. Then rub it smooth.
Add rub n buff over it if you like but it's not needed if you use the same cloth for the whole piece as bits of paint come off the cloth anyway.
I could do this again to get rid of some of the smudges but stuff it. It's not my costume. LOL
That's for flat surfaces and waethering that type.
For edgaes you just add shiny silver first then paint tacky paint on top of that to make it look like it's chipped.
I did experiment with sliver first then a glue paint mix after it's dry dry dry. Peel bits off to reveal the shiny underside. Worked ok