I removed all the undercoating, road tar and dirt from the bottem of my car in 3 1/2 hours with out even touching my car. Not one scratch or gouge mark anywhere. One afternoon and I was done.
Used a steam pressure washer with a little gasoline in a sprayer to soften the tuff stuff.
I've done it now on several cars and many trucks over the years.
It's worth the trouble/cost to rent a steam pressure washer for this job, imo.
I have been lucky to own several of these pressure washers that we use to maintain a small fleet of tanker trucks.

before

after same afternoon.




I removed all the seam sealer from the inside of the car also with it.