I've always known not to do it, but I never asked why. What exactly does it do to the paint?

 

Water dries too quickly, leaving water spots. Probably not the best for paint either. Really hot paint then hitting cold water can't be great for the paint (think Black car).

 

I always knew that fast drying = water spots... but thought there was a more serious reason for not washing in the sun... how bad, really, is it for your paint?

 

well, it's like this. i'm in the same boat as you in that i don't have some place covered to wash the car. what i usually do is either do it very, very early or very late. early usually works better here in AZ before it gets too hot and the sun is too strong. like around 6 in the morning. i wash it, continue to run water over it as i soap up the different panels, then when i'm done, do the lovely trick of taking off the nozzle and cascading the water over it to remove nearly all the water off the car. from there, i pull out my absorber, get the majority of what's left, and hit the street to dry off the baby with some wind. when i get home, i have my covered spot that i stick it under and pull out Z6 and detail all the water spots that survived along with doing the rims and windows and such.

i've been doing this for the last 6 months, so i can't see it as all too bad, as long as you take a few precautions like i have. here's a look in case you haven't seen her shined up before:

 

Yeah... I do one panel at a time, too... and I dry each panel with an absorber after it is washed...

 

for washing in the sun, I try to avoid water spots with a little vinegar/h20 solution and always use a polymer or acrylic

 

H2O/Vinegar would also strip wax. I would not recommend this often.

 

yep. i agree. i've only had to do this once, and the Z6 after the air dry does an excellent job of getting the water spots off before they can cook on the car. if you've reached a certain level of 'analness,' you'll have to detail your car to get rid of those pesky water spots or they start adding up real quick.

 

Water spots can also act as little magnifying glasses and we all know what happens when you hold a magnifying glass up in the sun. Conditions have to be just right but it can happen

 

water spots can be a pretty serious problem- depending on the chemicals in your water, they can etch your clear coat.
if you have ever had a water spot that you couldnt get out, you experienced the etch!
the only way to treat an etch is to compound it out (kinda like sanding) and then polish the paint to restore the luster. then, of course, the wax or synthetic finish has to be reapplied.

i would never wash a car in sun, not even my beater!