Since Astromechs are mechanics, hanging around the oiliest, dirtiest, grungiest parts of starships, it's only natural for these droids to stay clean for an average of 2-5 mintutes...tops. I had planned on weathering my droid from the very beginning of the project and any little mistake that was made in building has somehow been incorporated into the weathered look.
Nothing like a bit of "dirt under the fingernails" to complete the look.
I used Testors model paints (water based acrylics) and lots of water to create the "oil". Just slop it on and wipe it off. I had to do this quickly, hence no pictures of the process, but the results turned out really well.
More updates to follow, battery boxes are in process and I have two foot hoses completed. Still working on the back door, but I think for this event, I'm going to run with the door off (Artoo will be in a roped off area for safety).