Wet tank-top show.. Har-har..
Okay, here goes..
I have a military vehicle parked on my driveway (ex brit army Land Rover) painted in Nato green. The thing I noticed was the paint, when wet, goes a slightly darker shade, and is satin, opposed to matt. I suggest mixing up some satin clear with a hint of darkened base (i.e. same as the tank) and mist from above in the wind direction of your choice.. That way the airbrush actually rains on the tank, as it where. Depending on how long after it rained you might want to do less, and concentrate on more of the centre of the tanks panels (and steering clear from the engine area. If the tank has been running this will warm up the surrounding hull). If its just freshly wettened, then you might want to add some gloss afterwards. The closer to the centre of panels (or actually, the further away from the ground), the wetter. Puddles will form in recesses and flat horizontal panels. Also, water will have run down sloped/vertical panels, taking any dust and mud with it. If a hatch has been opened after it had rained, the standing water will have run off the side of the turret.
Test on something else, mind..
Also a dusting of gloss onto the diorama's grass will make it nice and wet.. Puddles are also rarely clear, and usually occur in the same spots, so they'll collect crud that washes in from around them. Some dark pastel chafings trapped by some clear gloss would do nicely I think. Also, has the tank moved since it has rained? It will be dry underneath, so the concrete will be noticably a few shades lighter. Concrete slabs also aren´t always level, so water will run off them forming a puddle in the direction to the way off.. I reckon after the base coat apply some darkened base 70-30 gloss-satin, then dust with satin then matt.. If its been some time since it rained, the slaby will be blotchy with certain areas being dryer. The further away from a puddle, the dryer.
Correct me if I'm wrong, fella's...