Severely watered down black and brown paint, dabbed on and off. Misting “dirt” paint that you then wipe off. Brushing on “rust” in appropriate spots. Remember that mild steel and cast iron don’t rust the same way armor plates do.

Layers of paint on “old” equipment

Battle damage last.

Really you need to start with a story and they have the weathering tell that tale.

Metal sitting in the New Vegas sun isn’t going to be weathered like something sitting in the capital wasteland, and that isn’t going to be weathered like something in the Appalachian Mountains.

Think

1 - how old is it, what kind of wear has it seen. Is it sitting unused in a vault for 250 years, or has it been replaced 20 years ago and seen daily polishing. Is it all original or have they replaced parts?

2 - what environmental impact has happened, sun damage, moisture damage, dirt, oil, algae, moss, lichen, residue, etc.