The cleaning is the most important thing to insure the stick'n...I also used 3M Weatherstrip
to seal all the holes and a narrow bead down the center over the screw heads, (you will find random mistakes under the moulding
). Brakleen, mineral spirits & green scrubby pads werk good.
Yes....seal the gap.
I also sealed the top edge on the sides, left the bottom open to drain.
Top & bottom on the rear bumper to seal the trunk.
I used Loctite PL