Just in the areas of where I drilled it for the mounting bolts. I then applied a small bead along the top edge just for aesthetics to keep the dirt out of the top seam. That way, If I ever need to remove it, (doubtful that would ever really be necessary,) it won't be so hard to get it loose from the trailer shell.
I use 3M-4200 sealant. I usually buy it in caulking gun sized tubes, but it is also available in smaller sizes, (like about the size of a toothpaste tube,) as well. Great adhesion and long lasting. I also used this a lot back when I did a lot of work on boats, and it doesn't peel off like silicone sealants and some other products. As an aside, the butyl tape mentioned above is also a good sealant that you won't have to struggle with if you ever need to remove it for some reason. It is the same product that they use to weatherproof and bed most trailer and RV windows
at the factories.