Same with caulking the roof. They recommend you clean existing caulk with a mild solution and then let dry completely. If existing caulk comes off easily then remove and re-caulk. If it does not come off easily, then caulk over it with their recommended caulk.
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Rick and Mindy
Retired
2013 Redwood 31SL, 2011 Ford F350 SRW, 6.7, Crew Cab with Firestone Bags, Titan Disc Brakes