Preparations for outside display
I once talked with a Lego Builder at the Florida park. He told me they use a glue that actually melts the Lego blocks together. With that in mind, I searched and found this article (on Facebook). It says Lego Park Model Makers


  use a solvent based plastic weld. It heats the bricks up so the edges bind together. Obviously it has to be very strong to withstand weather conditions.


A quick search on that found this from Google shopping.

The article also talks about color fading and model life time (about 5 to 7 years), after which the model can either be rebuilt or cleaned:


  We clean them using a shot blasting technique. They are sprayed with a fine salt spray, which takes a very thin layer off the bricks, and then they are lacquered with a UV coating. This gives them another 3 years or so.


I hope the project goes well.