We used to buy stuff similiar to what Rocco's talking about by the barrel for our fleet vehicles -- amazing stuff! I used it every year on my E36 altho it didn't get winter driven and it kept the body in amazing shape. This is honestly the undercoating of the gods.
After one winter and missing two years of treatment my car is showing some minor rust, I can not emphasise this enough to stop people from making the same mistake I did:
GET YOUR CAR UNDERCOATED EVERY YEAR WITH THIS STUFF. Rocco's deal is amazing, if it wasn't such a trek I'd be down there in a second. You won't regret it, the money will come back to you when you go to sell your car, keep the receipt too! For the 45 minutes and extra $60 over a regular oilchange this is a must-do for all bimmer owners.