Meanwhile I did some trials in Marc Reussers "Hairspray/Windex-Methode". What I want to try is replicating the color and texture of an aged sandstone wall.

Steps:

1. Heavily diluted mixture of Tamiya olive/gray/black here and there, mainly 
     in the darker top row
2. Clear coat with Future
3. Hairspray from can
4. Tamiya tinned mixture of gray/white/sand more or less everywhere
5. 30 minutes drying time
6  Windows cleaner shots to wipe away some color. Some color? Most of it 
     came off, even most of the Future!!!)
     So I hope with some further washes and detailing (e.g. lichen patches, ...) it 
     will become more contrasty.