They make undercoating remover.
Kerosene and diesel fuel works well also.
Kerosine and a scotch pad works like a champ. It's messy and you'll smell like kerosine for days but it's the easiest way to get it off. First remove the thick goop with a putty knife and then attack it with the kerosine.
Do NOT use a spray bottle, it will make the kerosine flammable (it won't burn redily in puddles or in a bucket, only with a wick) and when misted you will inhale it. Not good! I found it easiest to just soak some scotch pads and use that and a soaked rag for quickly applying some more and wiping off the last remnants.
Well, look at it this way, at least you didn't fire up a smoke or something while misting that stuff:flash: