We're going to make mosaics onto external ply using the direct method. The mosaics need to be okay to stay outdoors when complete. We're trying to decide if we need to use silicone glue, an external pva or other. We'd prefer to avoid using cementious adhesive on this ocasion. Any advice and any info about good deals for buying in volume?

i would,nt use ply imo , cement boards would be better, and why dont you want to use cement based adhesive, pva is a no no as no place with in the tiling industry

We want to use ply because these are individual works of art - like paintings - that will later be fixed up on walls either indoors or outside. The ply would be fine to crew mirror plates into. Cementious adhesive is less practical in a classroom and also you have to have a whole section ready to stick down. We want to be able to stick the mosaic work down one piece at a time.

right so these will be panels , you will take out side on various occasions

Yes, exactly. So what do you think - silicone glue?

So would a cement board not be ok for this?

I would still go with a cement bases adhesive, it doesnt need to be a rapid, could be an adhesive with an extended pot life to give you plenty of time to work with.

So if they are going to be outside, is there the possibility of these pieces getting wet?

Yes, there are a number of people working with us to each make their own piece of mosaic art and quite a number of them want to fix them up on outdoor brick walls in their gardens when they've finished.

We want to work directly because they're beginners and they might not all be confident to work indirectly and then install in situ directly onto their outdoor brick wall. Also, some of them will put their piece of work up indoors.

We want one method for everybody which is why everybody will make their mosaic on a ply panel. I've done this using exterior grade pva in the past (unibond do one) but I'm aware that other mosaic artists always use silicone glue in this situation. I just wondered if anybody out there had experience/advice about this particular application.

Trace

I'll let someone else advise on this as your proposed method is not the route I would take. If I were in your situation I would be using a cement board (doesn't deform when wet and is no heavier than ply) and cement based adhesive (which you can get slow setting with an 8 hour working time). Thanks for the post, I had to get my thinking hat on!

Trace, I'm with nybor and Rich, absolutely.

If you use ply and pva bits will fall off after two or three years at the latest, I know that from personal experience, especially if you are working with beginners where coverage is poor. Cement adhesive is no more awkward to use than any other, just mix up a little in plastic cups and give them out with little spatulas. I am also becoming more and more convinced that epoxy grout is the way to go for outside work, unfortunately we do not live in the Meds or any other hot country where things last so much longer, and bits falling off mosaics seems to be part of the charm there, not so here. If you want to know about epoxy grout I just posted a Mapei video that explains things pretty clearly

Okay - I bought some BAL mosaic fix adhesive today for another project, so I guess we'll try this for the panels too. Thanks - I'll keep my eye on those panels that we previously created using exterior PVA on ply. It was exterior grade ply with 3 coats of yacht varnish to the back and edges and we used a waterproof BAL grout. It was made last summer and still looks fine... might be going back to that job in another year or two, then!!!
Thanks for all the advice - much appreciated.