Oil paint rendering? Never heard of it?
I guess you must have missed the article by Mike Rinaldi, in which he in extensive detail explains his approach to the weathering of armour models. The article is simply amazing and I cannot recommend it strongly enough. Mike’s approach is illustrated using number of high quality photos and is well explained.
This article underlines Mike’s reputation as one of the most influential modelers these days and definitely one of my favourites.
The article was recently published in the Military Modelcraft International magazine (September 2011), but can also be found here: