+1 on the Dicor Lap Sealant. Been used on my cargo trailer seams. Also used it to install 3 Fan-Tastic vent covers on my 34WRS. Unless you are removing the old stuff, clean as best you can with a stiff brush and soapy water. then let it dry really well. If it is just spots here n there then denatured alcohol and a rag will clean it up.
Edit- Forgot to say that Eternabond is another consideration if you are doing the entire roof perimeter. There are a lot of positive results from members here.
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