all purpose weathering medium?

[brooklyn harbor rr fan]

Have finished a number of buildings for my shelf layout which are essentially background buildings.

All have colors that are appropriate, but look like the kits they of course are.

Can anyone suggest a paint (preferably water based) to render these structures with layers of commercial grime, but not to the extent of obliterating there underlying color or details?  I have acquired an airbrush setup and am in the process of teaching myself to use it.

A hobby shop suggested "jet exhaust", but when asked to produce it didn't have it and didn't know how to get it.  probably the tip was pased along from another modeler. 

 

[dehusman]

Pick any dark grey or blackish color.

Harbor mist grey (warmer)

Reefer grey (cooler)

Black

Weathered black

Any other dark color.

But also remember that there are grey dusts, reddish rusts, brown soil colors and bluish haze of distance.

[Graffen]

 + a wash of raw umber is a good and easy way to add some grime.

[MRRSparky]

"Jet Exhaust" is one of the Testor's Model Master line of paints, and I use it a lot for anything that is supposed to be metal but weathered:  e.g., brake line glad hands and air hose connectors; any galvanized metal (roof flashing, etc.).  It is a truly handy paint when used as a wash, such at 70% thinner (mineral spirits - relatively benign) and 30% paint.  I am surprised a hobby shop owner would not know of the availability of this paint.