Whoa....    the MSDS says that Dawn is NOT corrosive.  

Besides if it were, wouldn't it be advised not to use it on some pots
due to a corrosive nature ??

Snipped for brevity only - -

Since I already know from years of use and you are guessing and listening to old
wives tales, let us know how your nail test turns out.

Reality check, guys.

First, when you wash any car, with any type of soap, you are using a small
amount.  Seems like most instructions are a capful per gallon or such, so
what do you have, maybe an ounce or two per gallon of water?

Second, not all of these Sodium compounds are going to be corrosive to
metal. Without a chemistry background, you really don't know what they are.
Only Sodium Chloride (NaCl) do you know is just plain salt.  And it is
usually down the list so in one ounce of soap, you have what, maybe .1 ounce
or less.  And you don't know what the chemical composition will be after it
contacts the compounds in the dirt and grime on your car. They could all
become completely harmless within a few minutes of contact for all you know.

Then you are spreading that over an entire car.

Finally, you are diluting it further with not one gallon but lots of gallons
of water when you rinse with the water hose. (You do rinse, don't you?)

So your nail in a jar needs to have the 1 ounce of Dawn in a 5 gallon bucket
of water and mixed thoroughly. Then scoop your Dawn/water mix from that.
Then remove the nail after 20 minutes.  After all, when you wash the car, do
you totally immerse it in water and leave it for a few days, or is it all
run across the outside and then dried?

Dropping that nail in the jar and leaving it a few days (let's say 3) is
equal to 3 days x 24 hours = 72 hours / .25 hours per car wash = 288 car
washes.  Now if you wash once a week, that is over 5-1/2 years (5.538
years).

How many of you actually wash once a week?  Maybe once every two weeks?  So
that is 11 years of washing to equal the nail for 3 days.

Now, here is the kicker.  --- Is your car bare metal?  Or is it painted.

I don't know what started this thread, but I suspect it was something like
saying use Dawn or similar to wash the car initially that was found on the
side of a mountain to clean of the crud off of it.  I doubt there was ever
any suggestion to wash the car weekly for the next 5 years with Dawn.

When you pull a car out of a nasty environment where it has been stored
poorly for several years, sometimes you have to be a bit brutal in the
cleanup process to achieve a goal.  After all, when you paint a car, you use
water and sandpaper as part of the process to get it to shine.

I doubt anyone would say using sandpaper on paint was not brutal.  I doubt
anyone would say using Dawn one time was anywhere as near abrasive as that.

With Preston living in Wales, I bet the salt content in the air is more than
in the Dawn soap and water solution he will use on the car unless he lives
up on Mount Snowdon.

I'm SURE there is more salt content in the dirt and grime you get from daily
driving.