to undercoat or not to undercoat???

The word undercoating refers to many different products, some very good, others terrible. The old style undercoat was usually sprayed on the bottom of a car. It was asphalt based, causing it to shrink and crack in the winter. Moisture and salt get into the cracks, and before you know it, you've got holes. Oil based rustproofing became more common in the late 60's. You want something that can drip, so that it seeps into the same crevices that attract water. It's sprayed into all of the body panels, and if done properly, does a good job. If you're using your car in the winter, you'll have to re-do it every year. Unfortunately, nothing will protect the front fenders. They are poorly designed, and can't be rust proofed. The two inner fender panels rub against the outer fender, where any salt will accumulate, and cause rust. I removed the rubber gaskets from the inner fenders on my 69 Dart, but I don't drive it in bad weather. Montreal has the worst weather that I know of, for rust, and I can remember seeing these cars with holes in the front fenders, when they were two years old. In 1978 Chrysler started to install a proper inner fender in the Volare/ Aspens, but that doesn't help A bodies. I use Rust Check, in Toronto, but there are other brands as well.