Marine plywood cannot be used as a mosaic backer for outdoor and wet mosaic.
Yes, marine plywood can withstand the outdoors and wet days for many years, but it is completely unacceptable as a mosaic backer because it swells and contracts with changes in humidity in the outside air. That amount of swelling and contracting is tiny and might not be significant in construction projects, but it is fatal for mosaic. Absolutely fatal. It’s only a matter of time, and it’s usually not long.
People recommending the use of marine plywood as a backer for outdoor mosaics are not considering one critical detail:
The swelling and contracting of wood due to humidity isn’t trivial where adhesives are concerned, and the displacement (movement) can be measured. Imagine rainy days versus dry days. The displacement is more than enough to work glass free from adhesive because the glass isn’t swelling or contracting at all.
This is not speculation. I am an engineer and have worked in a materials testing lab.
Another piece of evidence I could bring to any argument about the use of marine plywood in mosaic is that I have received photos of tragically-damaged mosaics for 17 years, and marine plywood wins hands down as far as being the worst cause of grief, and the reason is simple:
Marine plywood SEEMS like a solid safe option because contractors will talk about the life they have gotten from it on certain jobs, and so the people who make the mistake of choosing it tend to be people who are making a design with a lot of work and care for the details. They took the time to choose a “good” backer because they knew they were going to put a lot of effort into their mosaic.
Seeing these mosaics damaged is much more painful than seeing some hasty work falling apart because the technical details were just outright neglected.
That brings me to an email I received from Monika Walter.
Artist Monika Walter
Monika Walter says she doesn’t consider herself to be an “artist,” but she has some solid work at her mosaic website, and she makes tables and mirrors and clocks for craft shows. They all look well-executed to me, and a couple of her mosaics make me jealous. More about that later.