Chassis Saver, is the best stuff I can find. I have tried some 3M undercoating from a spray paint can, I was not impressed. However, after two years of use, I am very impressed with Chassis Saver. No primer is needed, just paint over the rust and go, - no sanding needed, though I did anyways.
It is just very high grade undercoating. And it says you can paint over rust, and it stops it in it's path. That's what my car needs!
http://www.magnetpaints.com/rfi.asp Request a catalog and they will send you a 4X6 metal card with some Chassis Saver painted on it, I suppose bendable is good when it comes to undercoating.
I got a gallon, but that is far more than I could ever use.
CHASSIS SAVER™ Complete Pricing $USD UCP99-04 CHASSIS SAVER™ Gloss Black - 1 Quart 25.75
For thinning CHASSIS SAVER™ Rust Preventive Paint S8-04 Magnet Multi-Temperature Reducer - 1 Quart 7.00
Shipping was about $8, so for total of $41 you can get set up with tons of this stuff, with a quart you can defiantly paint your entire car with it, inside and out, so you have enough for you're entire underside, wheel wells, inside the floorboards, and any spots in the engine you so feel so to do.
MAKE SURE YOU READ THE Chassis Saver Disclaimer WHEN WORKING WITH THIS STUFF Not the usual Yada Yada
I use, or would have used this product for the following: