Fullers Earth is a very fine natural clay powder that the movie industry uses to create dusty and aged environments on sets, costumes and props. I used it on my proton pack to give it that dusty "Planet Hollywood" look.
I put some Fullers Earth in an old sock and used it like a powder puff all over the shell, then brushed it around with a dry paint brush...
It can be completely cleaned away with a damp cloth or be made permanent by lightly misting a clear flat spray. Since my pack just hangs on a wall, I'm good with it not being sealed but if you're a convention goer and want that dusty look to remain intact, then sealing it is a must.
If your pack is painted Semi Gloss black and you want to subdue the newness of the paint without it looking like it's been sitting in a warehouse for 20 years, cover your pack with Fullers Earth then lightly brush it all away. Avoid wiping it completely away. You'll be surprised at how well it brings out the details.
Fullers Earth can be purchased at Todd's Costumes for $3.99 per 5 oz. bottle.