OK; Rust-proofing/Undercoating?

Some food for thought;
Most cars that were undercoated back in the day have not experienced the rust issues that cars without did. I have picked up quite a few older cars and each one with undercoating has not had issues where the undercoating has been applied.
Another benefit is it cuts down on noise. You can use a product like Lizard Skin to cut down on noise and heat, but I am not sure how good it would be as a rust deterrent.
Andy