What constitutes rust? And why!

[DoctorSivana]

I have been reading the boards here off and on for quite a while. I really need to add my $.02 here. There is rust then there is SURFACE CORROSION, and the two, while similar, are different. Granted it gets down to nomenclature. But doesn't everything in the long run?
Way back when I went to school to be an A&P mechanic (aircraft, and no I don't work in the field), we had a full quarter devoted to corrosion control. It would be wise for many of you to look up some articles in the library about it.
Anything made of metal will have some corrosion, it is the nature of the beast. Go to a new car lot, get under a car and look. Wow, something that looks like rust. Nope, just surface corrosion. It is not bad, unsightly yes, bad no. There are parts of any car that will develop surface corrosion, usually exhaust parts (unless made of stainless steel, and even that will get spots of corrosion), and parts that come in contact with the battery and electrical system. Also spots where dissimilar metals meet ( ususlly there is a sacrifical joiner in a lot of situations like a piece of zinc) NEVER USE STEEL WOOL ON ALUMINUM, because you can cause irreversible corrosion problems) All are ugly but cleanable (if you really want to).
You can make an exhaust look good by painting it. But eventually the paint comes off and it's still ugly underneath. On the battery tray you can rubber coat it, and that will last a long time. But again, not permanent. Although it is a good idea to clean it out occasionally. That way you can prevent a return of problems for a LONG time!
Rust is a more pressing problem. Like most people recommend to bring a magnet and check for bondo, it is also good to have a probe (a metal tool like a dentist probe) to check "rusty areas". If you are on the underside of the car and it looks like there might be a soft spot (read: a rust through) scrape it with your probe to see if the metal underneath is good. If the probe goes through the metal...What did that tell you? Yep, don't buy the car.
Once a car has rust, it is going to have it. You can arrest the growth, but it never goes away. Cleaning will slow it, as will really good prep work before painting. But remember, it will come back.
I have owned literally hundreds of cars since I was a teenager (I'm 41 now) and rust isn't always insurmountable, just a problem)
Learn to recognize it and treat it and it will not be any more trouble than doing regular maitenence. Keep your car clean, and dry, and make sure that road dirt doesn't adhere to the nooks and crannies and it will last a long time.
A lot of this may seem pretty elementary to some of you (especially body shop guys). But I am amazed how many competent mechanics don't know about rust vs. corrosion.
If you have any questions feel free to write me at doctorsivana@earthlink.net