The way to do this is to use Gunk Engine De-greaser. Spray it into a contrainer and use a brush to work it into the undercoating.

Keep the area wet and use a poly tarp under the car. The undercoating will practically scrape off like butter.

I did my '66 Dodge Monaco this way, and I had previously tried all of the above mentioned methods, but none worked as good as this.

Brad
1966 Dodge MOnaco (Canadian Built)