still confused, what could possibly be in rainwater or coastal humidity that isn't in dirty dishwater? i get the focus is on salty water on the coastline, but i can guarantee the dish sink has been exposed to extremely concentrated salt solutions.
the difference is velocity. salt and dirt at 60mph velocity makes a big impact
I have no idea if this is the salient difference, but rainwater is actually slightly acidic -- it absorbs CO2 as it forms in the air, which combines with the water and turns into carbonic acid, H2CO3.
Various unfriendly-to-metal compounds free in the air (such as sulfur) that can precipitate out into the rain and thus onto the vehicle.
> i can guarantee the dish sink has been exposed to extremely concentrated salt solutions.
It also is usually rinsed after use, and sits dry in a climate controlled indoor environment 99.9% of the time.