What kind of glue/adhesive do I need

Usually clear silicone works (NOT Silicone II, the regular Silicone I adhesive).

 

Two part epoxies like Lords or ASI, JB weld will do, it's just expensive if you have a lot to glue.
Clear silicone works, but depending on the size of the letters, I'd want something more permanent. If it's 18" dibond letters, silicone would work well. 4' letters, there had better be about 10 pads, where if I was doing it with Lords, I'd get away with 6.

 

If small ACM, Liquid Nails PRO is favored by a bunch of my clients.


At 8" x 24 " I tell them to use fasteners. They still prefer LNP up to 18"x20".

 

Weld-On 45 is what we use. It is specifically designed to bond pre-finished aluminum and acrylic.

 

Boss brought back a roll of ... 3M? Red roll of grey tape, man that stuff is permanent. We spread a little backer paper, throw down a couple lines of this tape on top of it, and cut it into sizes we need. It comes easy off the backer once we've cut it. We use a lot of Lord's adhesive for bigger outdoor things, but that tape is monstrously strong. ...

 

Sounds like 3M VHB tape. (VHB = Very High Bonding I think.) Comes in a variety of types and thicknesses depending on your application. And yes, it is monstrously strong!
www.3m.com/3M/en_US/vhb-tapes-us/applications/signage/

 

Do watch out when combing this with silicone. We've had installers coat the tape because they put too much silicone down, and the tape only adhered enough to stay up over night. When I went to inspect it, the tape had no tack left, but a very fine coat of silicone over it.

 

agreed, do want to be careful about the adhesives touching.

LEXEL is another adhesive option to use