I have found that scraping it off after heating the undercoat from a heat a gun or even from a propane torch on a low flame held several inches away, (careful not to apply to much heat because of warping the sheet metal, also only use the torch in areas where fuel, oil or other flammable/combustables are not present, also you will need to have a fire extinguisher close at hand). To remove the very fine remants of the undercoating I sprayed a heavy coat of WD-40, letting it sit for a few minutes and it wipped right off with no hard rubbing required. Good luck it is a very long process but required for removing and repairing any rusted or otherwised damaged sheet metal.
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