I hate to be a bearer of bad news, GARDEN, but strandboard is made up of wood chips and glue. You preparations w/save it for awhile, but eventually the board w/expand in the weather. I bought a 4X8 sheet of the OSB yesterday for an inside art project - making/mosaicing a frame for a watercolor my DGD painted. Your thinset is the proper adhesive for outside. You'll get some time, I imagine, to enjoy your work, but it won't last a long time. I mosaiced a cedar pedestal, using thinset, about four yrs. ago, and it is showing no sign of deterioration, but I wouldn't use anything other than cedar for outside wood projects.

I would go get a pc of cement board to screw on it, or screw some wire lathe on it, and skim coat itwith mortar/concrete, then mosaic...would that work for you?

Well, thank you both...! I think I asked the question even tho I knew the answer...just in case I was wrong and hoping against hope that I was wrong. Calamity...great idea to put cement board on it and slowmedown I think you are right. I will have some time with it and then like a rose or an iris it will be gone. Lesson learned, but I have started and will hope it will last long enough to enjoy the result. Great thing is, I have not worked with flooring tile etc. so I am learning a lot. Having fun. Taking lots of advil cuz I guess I am out of shape.....lol. What a surprise. Next time I will make sure I don't put lots of work into a sinking ship, but I appreciate your words, cuz next time I will know (for sure) better. Many thanks.
Katherine

We're all learning here. :)
I'd love to see a picture of your Work in Progress!