I also say no to undercoating. Years ago (15-20), there was a big undercoating business in this area. (New England) The product names were Z-Bart (oil based material) and later "Rusty Jones" which was a paraffin based product. All the dealers provided it, and you could get it privately as well. I think it helped a bit, but there were other problems such as the undercoating getting into the door locks and so forth. They used to drill holes all over the place and squirt it inside of the panels, and then pop in little plugs.
Another problem was that it would block the passages and holes that are designed to drain water. For example, most car doors have small holes at the bottom so that any water that gets by the window can drain out. Block those holes and the water just sits in there.
Well, in the last 10 years or so, I don't think you can find anyone who does undercoating any more around here. Apparently it either does not work, or the cars don't need it. If you drive your car in the winter around here, it's gonna rust, no matter what.
1992 SSEI Supercharged
177k miles. 2010 GT coming soon.