Gary;

You are correct, rust progresses at varying rates as influenced by a whole bunch of factors. My statement: "rust at a similar rate at all the suseptable places" (note that I did NOT write "identical rate"), was intended to convey that if there is rust break-through evident in one of the typical places (around headlights), then there's a pretty good chance (I'd bet a hot fudge sundae), it is probably well along at some of the other suseptable areas, like transverse box sections under foot....that's not to say that you couldn't find exceptions...

-----------

I would like to comment on a couple of points you made.

Parking on concrete floors: In this park of the country, I'd rather park my car over a concrete floor ANY day than over an unimproved surface (soil, lawn, traprock over dirt etc.)...nothing is worse for an undercarriage than parking over a moist surface which indeed evaporates moiture, continously, at a fairly high rate. Your statement that a "concrete slab that sweated, profusely, every 24 hours", may only be valid in your area..."sweating" (or condensation on a surface) is a function of the temperature of a surface, and the RH of the surrounding air...sweating occurs when the surface is below "dewpoint", so this may or may not occur daily and is strictly a function of local weather conditions.

A concrete slab is a huge thermal mass taking on the temperature of the earth it is in contact with, and yes, it is porous, but the amount of moisture and the rate at which it is able to migrate this moisture through from below is nothing compared to moisture available from the air....the point is: The moisture which is condensing on the concrete under the car comes mostly from the air.

...but this is all academic...moisture is moisture, and if it condensing on the slab, it's likely also condensing on the undercarriage, and since moisture is a required ingredient for rust, whatever you can do to keep it from contacting the metal is good.

General: Making a barrier by coating (acid neutralized) surfaces with paint/undercoating, providing a sacreficial anode of a less noble metal - galvanizing, decreasing the RH to prevent condensation - park in a well ventilated, low moisture place - can you say sealed concrete or heated garage - or maybe moving to Arizona. Specific: Removing the moisture trapping sand which has been kicked up to between headlight bucket and fender sheetmetal in front, or between fender and frame toward rear, making repairs, and installing a barrier to prevent reoccurance.

My other idea: Repair using metal not suseptable to rusting (DeLorean had it right!).

"High heat" from the ehaust system doesn't not itself cause rust on the undercarriage...the exhaust system heat actually serves to cook away the rust producing moisture, not promote it. Exhaust systems themselves get hot enough to promote rusting sure, but the (much reduced) heat they transfer to underbody structure actually serves to cook away moisture...not such a bad thing!

Cheers