I have used Sikaflex to redo some leaking seams.  It works well.  I think my trailer's problem was the factory was too concerned about appearance and smoothed the sealant too thin over the joint.  I redid the joints two ways.  Wire brushed the original sealant completely out and just wire brushed the sealant's surface.  (used a wire brush in a power side grinder.  Anything less is too much struggle)

I finally chose to just wire brush the sealant surface clean and add more Sikflex over the joint.  Hint ... use masking tape on both sides of the joint.  Then remove the tape as soon as you've smoothed the sealant.   Don't wait for the sealant to set.  The stuff is tenaciously sticky.  Have a bunch of disposable gloves and rags handy.

Do you know the specific Sikaflex and color to use?  If not, do a search of my previous posts.