A lot of your weathered effects will be accomplished using a darkening "color wash". A wash can be a diluted mixture of paint and thinning solution (Mineral spirits or Water, depending on your paint medium), or a lightly misted layer of flat black spray paint.
For the Diluted Mixture I recommend going with a 5:1 (five to one) mixture of thinner to paint. When in doubt about color to gain an effect, try using black. You can wipe/brush this diluted mixture all over your prop in various stroke patterns to achieve various weathered looking effects. The ultra liquidly solution will naturally pool up in crevices and creases, making it look like the item has been around for a while and collected a good bit of dirt and grime. This solution can also be applied to a prop/costume quickly with an ordinary Spray Bottle, like those used for misting plants. In film production, a lot of on-site dirtying is done via the spray bottle method.
The Spray Can Dusting is an industry trade secret. To diffuse the intensity look of an object a prop maker will spray a lite coat of flat black spray paint over the item. When I say lite, I mean lite. Stand 3 or 6 feet away from the object and spray, holding the nozzle no longer than 2 seconds at a time, until the desired effect is achieved. It does not usually take more than 2 mistings to darken and diffuse an object.
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