Maverick and I noticed the same thing yesterday on my 06 GT Vert when he was putting my Borla Stingers on. I have less than 500 miles on the car. I attributed it to the 25" of snow I got last weekend, but now I am leaning towards otherwise. My whole rear axle was rusty, not just the part you see. You'll see if you crawl under the car.

My pulley is also one big red mess, but that's being taken out and replaced with a Steeda set, so I don't care.

Hopefully you'll get an answer so I can follow suit.

 

like far too many cars, the mustangs rear axle and driveshaft are not painted, nor the aluminum engine...aluminum might not turn orange like rust, but it still oxidizes and looks like crap after even a few months of humidity, or a trace of saltspray.

I tore my car apart with 300 miles on it, to undercoat the hard way, and theres a lot of areas that should be painted...the lower front control arms are a steel laminated weldment, with punched holes- the holes let water between the layers, and that starts to rust. the nuts on front brake hoses were already rusted at the threads, numerous bolts underneath(look under rear seat area at the bolts holding the flaps in front of fuel tank- mine the bolts are rusted so bad you'd swear they were ten years old), the driveshaft/carrier bearing housing/rear axle...just as bad is the engine block/tranny case, crossmember...when you get your customer survey, be sure to attach a letter to Mr. Giombetti, and suggest they start painting everything, like they used to. I put up a thread over at blueovalforums.com asking for engine appearance upgrades, like they offer for interior- to some goofy folks like myself, I'd rather pay extra for a better looking engine than leather seats...it could be a moneymaker for Ford. I do plan on dropping my driveline/fueltank/vapor canister cover/exhaust system and heat shields this summer to undercoat everywhere else(I used roll on bedliner, and POR-15 paint with a syringe/tubing to seal all vertical/bottom pinchwelded areas). new cars are galvanized, and likely not to rust thru for 5-10 years even without much care, but I tend to keep cars much longer- this being my alltime favorite machine, will likely be passed on to my kids when I'm long gone- hoping to seal up well enough to allow that- with still driving in all but nasty weather...its been rained on, driven on dry salty roads, etc, but I'll be darned if I could let it set 1/4 of the year. It wont last forever no matter what, and neither will I, so might as well make the most of it, while still trying to protect the investment if possible.

if youre worried about rust(like me) you might want to look below at the 'might want to look under your car' thread- lots of pics I put in there of areas that concern me infinitely more than the cosmetic rust on the axle...I spent a week of evenings under mine to get it mostly winter ready, and feel its good enough to protect from most of the salt entry points while still allowing everything to breathe, but plan on much more work underneath when weather/longer daylight permits. Hopefullu by next winter I can be confident that even driving with wheelwells solid with slush wont harm anything(not that I would, but you know what I mean...peace of mind)

 

I've noticed this when my car came off the transport truck. I use a rust encapsulator and then some Eastwood chassis black. I'm lazy and don't want to totally clean off the rust...prime..... then paint.

 

The rust on metal parts is normal, it doesn't hurt anything. Unless you havea show car that gets judged on how the underside looks, you are wasting time and money painting those parts.

 

A lot of trucks and commercial vehicles are like this too. They are engineered to get a surface coat of rust and they stay that way for many years with no deep penetration. Totally safe, albeit ugly.

 

I can't take all that rust either. So, since my car is in storage til the roads clear (they sand with hugh red volcanic cinder rocks here in Oregon) I put it's rear end up on the ramps and went to work on the rear axle. Primed, and then 2 coats of satin black. Looks great! I'm not thrilled about the six hours of cumulative painting time that Ford should have done though. How much did it save them? Twenty bucks, I'd gladly have paid.