I think that the term 'overweathered' needs to be broken down in different segments.
Paint chipping
When applied this is usualy overdone, using reference photo's from many sources shows very little paint chipping, other than abandoned/badly damaged vehicles. Paintchips are something which will be attended to during routine maintanance, and makes for an interesting (although difficult to replicate in scale) effect of patched paint work.
Mud and/or dust
This is often underdone, a tracked vehicle rips up the ground like a plough, and flings it everywhere... AFV's will be coated in mud/dirt shortly after moving off, and contrary to belief, would not be washed at regular intervals. I mean where did the mobile vehicle wash come from? And washing a 60 ton Tiger ( or 45 ton Panther or even a 23 ton Pzr IV) with a bucket from a tap? Sure, when essential maintanance/reloading takes precedence washing a vehicle is the last thing on your agenda.
The trick is to create the coating of mud and dust, which will partialy wash of in the rain, covered again, layer after layer. This is where washes or light coats with the airbrush come in.
Dust, unless it rains, an AFV will trow up lots of it, and be covered within miles of moving of.
The only photo's I have seen of crews washing the whole vehicle are either in a rear maintanance/refit depot, or parked in a river.. Don't forget, mud and dirt are also another form of 'camouflage'.
Rust
Oh rust, don't we all love to recreate that '10 years in the junkyard look'? :-) I'm guilty of it myself, I do it with the exhausts..

A vehicle in continuos service will hardly rust at all, again there would be a specific reason for it. Tracks will rust over night, but this rust will disappear when the tracks start moving again. Rust streaks down the super structure? Not unless the vehicle has been immobile for quite some time. Rust, like paint chips and unlike mud, would be attended to at every opportunity.
Damage
Damage can be tricky, in general you should balance dirt with damage, i.e. a badly mauled vehicle would typicaly by filthy, whilst an undamaged vehicle has probably not yet build up the multiple layers of grime. Unless the crew has been very lucky/carefull..
Oh well, time to get of the soap box and put my own words to use.. Tonight is weathering night, I havea few kits that I'm working on tonight, just weathering.
EDIT- Just realised that my old bias towards German AFV's slipped in again.. I have of course seen plenty of photo's of filthy Shermans, Churchills, M10's etc -EDIT
Cheers
Henk