Gently heat the opposite side of the panel where posible,
I use a propane bottle with a blow lamp, with a sensible flame, then the underseal scrapes off with, as you said a scraper.
The re-treatment is up to you, as what you find as the best looking ect.
My sugestion is to shults, as you will more than likely take off some paint when removing the original underseal, and wax oil tends to be more translucent, but I guess you will see and deside as and when.
Hope this helps,