A quick search online turns up a number of pages that recommend against giving your vehicle a carwash in the hot sun.
Will this actually damage the finish on the vehicle? Will it cause any harm to the car? Or is this just a widely perpetuated myth?
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My understanding just based from experience is it means its easier to clean because water doesn't evaporate in 10 seconds and leave those dirty residue spots on the car.
Its easiest to wash just before sunset when the strong suns rays are gone and temperature is cool and then you've plenty of time to chamois the water away for great clean finish.
Then its time to cruise the street and admire your cars reflection in shop windows.
The thing about washing it on a hot sunny day is the soapy water dries quickly and the soap spots can be a pain to get off, or at minimum look ugly when you're done (which often defeats the purpose of washing it in the first place). Also water spotting especially if the water in your area is pretty hard. Not permanent, but still a pain.
I guess there might be some types of finishes where soapy residue (or certain mineral deposits from the water) left on them for a long time could damage or become more permanent. This is probably one of those things that like, might be an issue, if you do it all the time and are unlucky. I wouldn't worry about that. I've never heard of anything like that ever happening, and I can't imagine a situation in which it would, or the chemical mechanism behind it.
It's not going to like, strip the paint off your car. If people are saying it's going to damage your finish I'd say that's a myth.
If you have to wash your car on a sunny day try doing it in the early morning or late evening, or in the shade. You could also try to find a gentler, more pH neutral shampoo. The other thing that helps is to wash your car in smaller sections (shampoo -> rinse -> dry) so the soapy water and rinse water has less time to evaporate before you dry it.
All in all I wouldn't worry about long term finish effects, but your car may end up with a lot of unsightly soap spots on it if you let the soapy water dry before you rinse it, or water spots if you let the rinse water evaporate before you dry it.
Anecdotally, I have washed a lot of cars, and seen a lot of cars washed, and in the last 30 years that I can recall, I have never once seen an issue besides sometimes having to re-wet a spot that dried too quickly.
It's sort of like if you spray glass cleaner on a window then let it evaporate before you wipe it off. It's annoying, doesn't leave you with a clean window, and makes more work for you.