There are plenty of "acid free" boards with pressure sensitive adhesive already on them. I used them on RC prints in the past. It is best if you can use a roller station or vacuum press to create a lot of pressure. Even though it was "acid free" it shortened the life of the RC paper to only a few years. I kept samples to see the long term effects while I was working at that studio. Degradation began to show after 4 years. I have used sprays, but they simply do not adhere well at all. Seal makes a film called Optimount for adhering prints to Plexiglas that is supposed to work well, but again, you need a good amount of pressure to get good adhesion.