I skimmed over the responses and it seems like you have received a lot of good information. Really boils down to what you're trying to achieve.



With that said, I know of two "somewhat" new alternatives to the grout and silicone/coco-fiber methods.



Some people are now using Drylok Latex Masonry waterproofer instead of grout to coat expanded or extruded polystyrene backgrounds. Easier to work with and more durable is the word from people who have tried both methods. Use liquid concrete dye to tint the Drylok.



And instead of silicone try an acrylic concrete bonding additive. This is been in use in Europe for a while and now more people are trying it. Some create a slurry of coco-fiber and bonding additive and smear it on the background. Others use a garden sprayer to apply a thick layer of bonding additive and then sprinkle the coco-fiber in place.



Don't know if these were mentioned or not but thought I'd mention them just in case.

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