I'm pretty confident that simply spraying water on stainless steel won't cause rust. While stainless steel is resistant to corrosion, it's not completely impervious to environmental factors like chemicals, rail dust, and pollutants.
These substances can coat, degrade, or stain the surface, just like they can with paint, chrome, or other materials.
The stainless steel panels on the Cybertruck don't have a clear coat. The whole point of using stainless steel was to eliminate the traditional exterior painting process, including the clear coat. Stainless steel actually performs better when exposed to air. The chromium at the surface oxidizes, creating a protective layer that resists corrosion. However, this layer can be vulnerable to certain corrosive substances, like bird droppings, iron-laden dust, acidic rain, airborne chemicals, and salt spray.
Because the stainless steel panels were work-hardened to make them more durable, their corrosion resistance is slightly reduced.
I think the Cybertruck experiment will ultimately lead to one of two outcomes:
Owners will apply a protective wrap to maintain the surface and achieve a uniform color.
They'll learn to accept the stainless steel's natural patina, including fingerprints, rust spots, and all.
I don't think we'll see another EV, from Tesla or any other manufacturer, use stainless steel in this way or attempt a similar "Cyber" design.
The Cybertruck, like the DeLorean, is likely to be a one-of-a-kind model with a lot of potential but too many flaws.