True - Plastic putty knife is a great tool. Reduces likelihood of tearing the roof liner. Warm day helps too. IF you are really careful a heat gun will soften it. I don't think it's necessary to remove every trace of old caulking. The new stuff will be soft and pliable and cover/seal small bits that remain.
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2012 FunFinder X 210UDS
4-68 watt flexible panels; 1-215 glass panel/4 Battle Born LiFePO4 Deep Cycle Batteries
2014 Ford F350 6.7 Liter PowerStroke V-8 Diesel longbed, SRW