I've wondered about this for a while. When you look under a new car, you see that everything is either painted metal, plastic, or aluminum with its own bright finish - except the rear axle/differential case on RWD cars and trucks. It looks like they don't put any kind of finish on these parts because they form a rusty surface appearance almost immediately. Why? Is it because of the kind of metal that an axle housing is made from? Cast Iron? Steel? I'm pretty sure those surfaces can be painted. GM doesn't paint the whole assembly but on my new Mustang, I see that the differential case seems to have a black paint on it but the left & right axle tubes are bare - and rust already setting in. I wonder why they're not painted or at least coated with a protective finish while almost everything else is.

 

The rust in itself will become a protective coating, but I have no idea why they didn't just paint the friggin tubes.

 

typical penny-pinching. 'nuff said

 

I have a Take-off 8.8 Rear on the way for my V6 ... It will be painted prior to installation to prevent it from rusting. Will probably clean up the drive shaft while I'm at it ....