This may sound crazy but Brown and black liquid shoe polish works great for weathering props and it works on most types of surface and materials i use it all the time for my replicas,you just need to give a light coat of hair spray to stop it from rubbing off once your happy with it.On mine I spray them h a red oxide primer, then I apply the gold leaf fully to the inside and randomly on the outside. Next I dry brush the entire exterior of the cup with some acrylics that I mix. Afterwards I apply a mix of plater of paris, portland cement and sand that I've crushed very fine with a mortar and pestle, basically the same ingrediants used in ultracal 30, I just use a different ratio.
The original grails were primed in a red oxide color, cherry red paint applied, gold leaf applied sporadically on the outside, then had ultrcal 30 applied and they would then rub some off until the desired look was acheived.
I've never had a problem with hair spray that why I now use it as your right some car clear lacquer spray reacts with it, it's happened to me in the past.Careful using shoe polish to weather. Not only does it contain dye (duh) but solvents and oil. If you're going to coat it afterwards with a clear coat, you may have problems.