Use waxoil not underseal ;) or better still mix underseal with waxoil
Underseal dries hard over time, and if you chip it you'll get rust growing nicely away to itself protected by the underseal.
If you mix in wax oil then it stop's it setting and it becomes self healing.
If you've access to a compressor and one of those parafin blowers, waxoiling is pretty easy to do. Get a length of hose pipe though so you can do inside the chassis rails through the holes. Most things rust from the inside out ;)