Wurth sells a tool that grinds off the undercoating , its like spinning teeth that chews off the undercoating.
Might not reach in all the cracks but saves a lot of time scrapping.

My body man told me he paid a little over $200.00 for his tool. He uses all the time in his shop to remove undercoating inside doors, floors, etc on his clients vehicles, saves him a lot of time scrapping