There is no exact science to it just as nature has no exact science as to how dirty it can make an object over time!

Just like nature makes things dirty I can do the same with models using what I tern as 'dirty water'!

That dirty water is just brown paint diluted with water to somewhere 1 in 5 to 1 in 10 and a few drops of IPA AKA rubbing alcohol added to break down the surface tension of the water.. There is no exact science to it just as nature has no exact science as to how dirty it can make an object over time but by brushing a coat or two of the dirty water on a model and allowing gravity to work you can replicate nature. Just got the five chassis to do and the five superstructures!