For a really long time, I’ve always assumed that leaving your car out in the rain, especially when it’s raining heavily, is the equivalent of getting a free car wash. Turns out, that’s not true at all. If anything, rain makes your car dirtier than it was before.
First of all, rain falling from the sky doesn’t have enough pressure to knock off or remove the dirt that’s stuck on your car. To really wash your car, you need to use a bit of soap to release the dirt particles so they can be washed off.
Secondly, rainwater isn’t exactly clean. Depending on where you live and the amount of pollution in the area, the rainwater can be rather dirty and even acidic. This means over the long term, they can damage your car’s paint or coating, and this is apparently one of the reasons why people wax their cars.
So, the best thing heavy rain can do is reshuffle the dirt around your car and that’s it. If you don’t want to get a car wash, the easiest thing you can do is to manually spray water all over your car, give it a quick scrub or wipe, preferably with some soap, and rinse it again. Which, if you think about it, is the equivalent of giving your car a quick shower instead of a full bath treatment.