I would caulk the exterior corners, then trim with 1"x2", then caulk again. Keep the water of the edge of the sheet. That is how I do mine and they have stayed water tight.
BTW, make sure the trim pieces offset the sheet to break up the seam.
Edited by Western (05/22/15 06:29 AM)
“Never be afraid to try something new. Remember, amateurs built the Ark, professionals built the Titanic”
Friends don't let Friends drink and post.......
The first 5 day's after the weekend, are the hardest....