Dousing your car with water in bright sunlight can be
ruinously bad for its finish. First, the sun heats up the metal to the point
it's scalding hot to the touch. Then, you add some nice tepid tap water --
which is immediately boiled off the finish, along with caustic soap and
whatever grime you didn't clear off -- while any remaining rivulets of moisture
act like a magnifying glass, amping-up the power of the sun's murderous rays.
The end result is not pretty -- and cosmetics aside, you've cut into the
resale/trade-in value of your vehicle by flailing its finish.
New cars with base/clear-coat paints are especially
vulnerable to sun damage, because once the very thin clear topcoat is burned
away or otherwise damaged, the paint will never shine again, no matter how hard
you wax and polish. Only an expensive repaint will fix things. It's far better
to wash on cloudy, overcast days -- or at least in the shade, away from direct
sunlight. A great time to wash a car is just after dawn -- and in the late
afternoon, just as the sun is slipping past the horizon.