I like to describe my weathering methods as 70% of the results for 30% of the effort. There are many people, especially in the plastic kit modelling world, who do a better job. The effort required for their models is tremendous and most of the time, I can't be bothered. Let's face it, even the idea of spraying everything puts me off.
Anyway, the tubs are done and (I think) look OK. After the woodwork was painted, all the metal had a coat of Humbrol dirty black acrylic, which weirdly gummed up a nearly new brush destroying it.
Then some washes of black and brown Citadel weathering washes. Finally, loads of powder. Inside, there was a shot of hairspray first to ensure a really good, claggy buildup of "coal".
The finishing touch was rubbing a normal HB pencil in the wheel treads, which looks rather effective.