Sorry if I am getting my wires crossed here and maybe I am miss-reading what you are wanting to do.
If you are installing a folded pan composite sign onto a similar style shopfront. I would not use adhesive at all mate.
Your primary fixing points are the screws you use to attach the frame for the pan-sign to be fitted to.
For this, I would recommend Aluminium angle and 75mm concrete screws. Similar to the illustration I have attached.
Once the angle is fixed in place using the concrete screws, simply slide the pan sign over the trim.
You then use rivets or screws along the top, bottom and side if required.
Using the likes of concrete screws will ensure a solid fixing and you will not need as many as you would if you were using something like screws and rawl plugs.
If you haven’t used a concrete screw before.
Make sure you have the properly sized masonry bit for drilling the hole.
Make sure you also have the proper torq bit or tech screw head for your drills bit.
Drill a neat hole, not a loose or a slack one. then just screw the screw in at a slow steady pace. the screw will cut its own threads into the concrete.
Test using one on your workshop wall or similar. leave a bit of the screw head proud from the surface and try using a claw hammer to yank it out. there is no chance if done correctly!
With this in mind, you are fixing a lightweight aluminium angle frame to the wall.
your then riveting or screwing a lightweight, soft aluminium composite box to the frame.
I can guarantee you the pan sign will be torn and destroyed, and the aluminium buckled or snapped, before one of those screw fixings into the concrete will budge.
if drilling into sandstone walls, make sure the concrete screws have wider threads because sandstone is softer and can crumble or thread if done wrong. a quick test before starting is always recommended.
As for adhesives used on the composite.
Some two-part adhesives really heat up when they are curing. so when you put a blob of adhesive on the rear of the composite. you can actually see at “certain angles and lighting” dimples or uneven areas on the face of the sign, and looks bad!
So it is always better to test your choice of adhesive with a scrap bit of composite and see if anything happens.
However, some composite sheet materials being sold are not of the best quality.
This normally means the aluminium skin on either side of the polyethylene core is very thin.
At times even the core is poor quality…
As a result, sometimes using adhesives that are not necessarily as “hot” as some, can still create adverse effects on the face, when applied to the rear. again, doing some tests on scrap materials you use is advised. all i am talking about is putting a blob and a line of adhesive on the rear of an offcut, leaving it to cure and then take it out into the sun and walk back from it and view at different angles. try the same when the sky is cloudy. as the lighting great effects when it can and can’t be seen.