You guys may want to try this. I used it under the hood of my '40 Willys and the whole underside of the car.
Body Shutz!
It's a rubberized coating like an under coating, but it easy and cheap to apply.
You can get a quart of it and the gun for under $50 US at Amazon.
Amazon Body Shutz This will help with the flying gravel/stones on the underside and also protect from rusting as quickly.
