would have said SMP too mate…
Personally, we put out a ton of panatrim signs each month.
i build them completely in-house and cannot be doing with faffing about with clever hidden joins. tongue and groove type joins, foam stick, clips, spring tensioned ends and more… they all work to an extent, but in bad weather and over time they open, look slack and more. i now have a hidden joining strip down the back. but i put a minimum of 4 rivets "thru the face" 2 tight to top of frame 1 inch either side of join, and 2 tight to bottom of frame, 1 inch either side of join.
from the ground the bottom ones are pretty much invisible, were as the top too can be seen but very neat and tidy. i have never had a single complaint about their presnece.
depending on the depth of the panel there maybe another 2 midway also.