Back in the day(pre 1975) when we worked for 3M painting billboards we would on rare occasions get a work order to use 3M scotchcal,..it was a liquid based adhesive(sometimes tinted alot like the modern day urethane clears) and the reflective beads that you poured over the adhesive like smalts. It isn't made any more and Minnesota Mining and Malfunction has discontinued its production,...seem to recall something about lightning being very apt to strike any wooden structures with the stuff on it,....it made for some interesting burnt up signs along the expressway