In thick areas of under coating, I used a propane torch. When it wasn't that thick, I used a heat gun. Just keep whatever heat source you're using, just keep it moving and you should'nt have any issues with warpage. I also kept compressed air near me at all times. Whenever I felt the quarter panel beginning to get warm, I'd shoot some air to cool it down. Once it was all off, then I used some paint stripper. Scraped the last remants of paint and under coating off and the wheel well areas were ready for media blasting. OEM panels are more thick steel gauge than the repros. Remove under coating out of wheel wells with the quarter off if you are planning to replace the quarters.