I have been researching tools for removing the undercoating.  I have narrowed it down to 2 tools:

This is one: Dent Fix DENDF701 Metal Blaster Mbx Kit
You can resharpen using a grindstone and running in reverse.
Course wheel for undercoating
Medium  wheel for heavy rust
Fine wheel for paint

Here is another idea:

I plan on giving both of these a try – luckily, I only need to work on the wheel wells and not the entire car!