It's already been mentioned but I would start with dusting the idol with a dull coat (don't make it too even). Then, I would age using oils mixed with turpentine or naptha.
Humbrol makes great weathering powders. I know they have a chrome oxide if you want that Bronze effect. I tend to dab the area with a wash colour from Vallejo and apply the powder to stick it.
Yeah! I usually use the "wash" paint from Vallejo, dab a little of it, and before it dries, use the powder, as it helps to set it! No idea if this is the way to do it, but it's the way that works for me haha! I have used this technique to make the rusty effect on all of my Fallout props, and did the trick nicely