Just as an overview, I think realistic looks have "noise," and "dirt."  There is a fair amount of overlap, but think of the former as being overall iregularity, and the latter as being specific spots.  Or the former as being amenable to procedural methods, the latter as being better-suited to texture maps.

 

Take a windshield.  All of it will get scratched, dusty, and water-spotted (more-or-less).  Grime will collect in the corners -- which can possibly be done with advanced procedurals.  But the streak and scratch patterns of the wipers are specific to where those wipers hit, and are best handled in paint.

For a nicely-used Army jeep, I'd try using a procedure to bleach the paint on the upper-facing surfaces and generically add dust on the lower parts.  And add a general procedural for the small-scale irregularities of the paint/surface.  But combine these with a rust-and-grime map that paints in where mud splashes from the tires, where oil drips, where boots scuff getting in and out, and more wear/scratches/bleaching on the higher traffic areas.

Or just one big hand-painted map, plus an additional noise procedure to add a little fine-scale noise.

 

Read up in any plastic model magazine/forum, Fine Scale Modelers et al, or the equivalent in the Model Railroad community, for how they approach what they call "Weathering."  Physical props people -- my favorite blog right now is Volpin Props -- also go into the techniques of weathering. 

 

Which us theater people used to call "distressing."  Still call it that.  I'm rambling.  Starting up on texturing some steampunk stuff right now and that's a whole load of grime and patina and so on.