Water spotting is one thing, etching is another.
What one may think is a "water spot" may actually be "etching"
Real, actual, water spots are nothing more than the minerals in the water or on the surface-exposed to not enough "flushing action" by rain, etc and are allowed to "dry" on the paint surface and those minerals "bond" to the porous paint film. Paint, be it a clear coat or a single stage, is very porous, not seen by the "naked eye", it looks to be solid, not so.
Any paint surface on a vehicle may absorb up to a pint or more of water into it. This water will evaporate under the exposure to heat that they are subjected to. Sun, heat gun, wind, etc.
Any of the many acids present in the enviorment of which the vehicle is present, will liquify and and enter the paint film. That, combined with "heat" activates the acids and they eat apart the all important "film former/binder" of the paint. Once the binders are eaten at, they are toast.
No clay bar or such will correct the damage.
And, whatever acid based chemical that "ate into" the paint may only be removed from the substrate of the paint film, through the use of OEM (vehicle manufacturer) tested and approved by them. Not by some "gee this is simple and great, buy it" product that is marketed to the uninformed.
Only one system is capable of doing this. This system is private labeled for several manufactuers or required in their "dealer issued" Technical Service Bulletins", of which several may be read at ValuGard.net.
Once the "damaging acids" are neutralized and removed, then one may attempt to remove the damaged paint film.
However, if is doing so, one removes more than .3 mil (3/10th of a mil), the all important UV blockers, which are concentrated in the top .5 (1/2 a mil) are removed, the finish has lost it's true "density" and is very porus and not enough UV blockers are left to truly protect the total film build that is on a vehicle.
Clay bar, buffing, only removes the surface damage. It "does not" remove the actual acids that created the damage in the first place.
It's really basic chemistry, having to do with "heat and moisture" reactions of acids, moisture and heat.
Go back to your schooling of how these things work and it may become more clear.
It's all there, on that website, just up to each to make a decision to read and take some time to understand that it is not a "mass marketed, look at my pretty packaging" site or information.
If one is serious about understanding what and why, I suggest they take the time to check it out.