Mosaic Street-Number Sign by artist Monika Walter damaged

Warning: Marine Plywood Not For Mosaic

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.

The first thing to know about Monika’s work is one of the reasons why she takes so much care with her andamento is that her husband is blind, and he gets to appreciate her work by feel.

The worst part about it is that Monika’s husband was the one who recommended using the marine plywood.

In his defense, they do live in a desert climate. And the thinset she used wasn’t Versabond, and it might not have been sufficiently hydrated.

And contractors in the forums do talk about using marine plywood and just putting Versabond right on top or regular plywood for quick and dirty backsplashes and stuff like that.

But you have to remember that a backsplash isn’t getting exposed to wet outdoor conditions from the backside like an outdoor plaque would, at least not until the house settles or the face of the mosaic gets damaged or a pipe leaks.

The benefits of using stone or tile backer board instead of plywood are obvious if you consider the mosaic below:

A Tale of Two Mosaics

Dream in Blue Mosaic by artist Monika Walter
Dream in Blue Mosaic by artist Monika Walter

Monika tells us that the Dream in Blue mosaic above was made on concrete backer board and has been exposed to the elements of southern New Mexico (heat and relentless sun, torrential rainfalls during monsoon season, and occasional sub-freezing temperatures during winter) for years without even the slightest crack in the grout.

For the address sign that started failing, she used marine plywood, and the pieces of glass began popping off after only 8 weeks.

Note that the damage started at the bottom of the mosaic, a clear indication of water absorption being the root cause of the failure.

Color Family Harmony

I am jealous of the Dream in Blue mosaic above.

I am jealous of how Monika used a harmony of related hues and how they go from violet to blue to to cyan to turquoise green to the yellow highlights of the iridescence. I like how the iridescence expands the range of hues at both ends.

I also like how the stained glass shapes are incorporated with concentric swirls of smaller tile and how there is some figure-ground reversal going on. I’m a little jealous of that and how the stained glass composite shapes don’t all use the same scheme size/shape for the pieces in it. Each is different. That makes the mosaic more interesting.

Monika says that all the materials in this mosaic came from us, except for the glass gems.

TIP: Many glass gems for crafting aren’t meant for traditional mosaic because that are colored plastic on clear glass, and the grouting process is usually enough to scratch the plastic. You should test gems purchased at craft stores by cutting one up or scratching it on a rough concrete surface. Use gloves.

Mosaic Mirror Joyful Reflection by artist Monika Walter
Mosaic Mirror Joyful Reflection by artist Monika Walter

Joyful Reflection

If you wanted to make an abstract mosaic of archetypical shapes and borrow from ancient Egyptian hieroglyphs and their color scheme of gold, carnelian, turquoise blue, and green, and you wanted to make sure you kept it abstract enough, I don’t think most mosaic artists could have done a better job than Monika’s Joyful Reflection mosaic mirror.

The motifs in this mosaic aren’t copies of hieroglyphs. Instead they are abstracted interpretations of the colors and shapes used in the hieroglyphs. Or at least that is what it looks like to me.

If so, this really is a good example of using traditional archaic art to inform and inspire contemporary art. It reminds me of how the art of Walter Inglis Anderson was heavily influenced by Adolpho Best Maugard’s book Drawing Method: Tradition, Resurgence, and Evolution of Mexican Art, which explains the seven elements in drawing and discusses how archetypical shapes in nature (spirals, wavy lines, etc.) became the archetypical shapes in primitive art.

I like the flat colors of the shapes and how that contrasts with the mottling of the gold background. The happy bright colors that catch the eye, and the mosaic is aptly named.

My Recommendation

My recommendation for salvaging the street number mosaic was to dry it out thoroughly and repair it. I would then paint the sides and back with multiple coats of an outdoor paint (oil based). Then I would relocate the mosaic underneath a porch or better still inside.

Alternatives to Marine Plywood

For an outdoor mosaics, there are better alternatives ranging from light-weight foam-core boards to heavy concrete:

The foam-core board would need something to cover the edge, but so would marine plywood.

13 thoughts on “Warning: Marine Plywood Not For Mosaic

  1. Paj Tognetti

    Thank you for your informative article. I am concerned about all my various ‘glass’ gems. I don’t quite understand how you would know by scratching the gems if they were made of glass or plastic?? I would imagine that both materials would scratch? Could you please explain the process and perhaps include some pictures to see the difference?

    Thank you!
    Sincerely,
    Paj Tognetti
    boborabbear@gmail.com

    Reply
    1. Beverly Roberts

      I called laticrete and they told me I can’t use hydroban board for exterior use. Did I speak with the wrong guy?

      Reply
      1. Joe Moorman Post author

        Hi Beverly,

        You might be right. It might only be for indoor wet mosaics. I am researching and will edit the article.

        Thanks for the heads up!

        Reply
        1. Val’s Blank Canvas

          I use cement sheet for exterior mosaics, same stuff used to line wet areas inside houses. . .showers, bathrooms, laundries, etc. after cutting the shape requires. I cut it with an angle grinder, and always do that outside wearing goggles, mask, and ear protection. Dust is not as lethal as asbestos fibres but still nasty stuff.

          Cement sheet can be nailed with galvanished cleats. After fixing to whatever, coat with waterproofing compound . . . again same stuff used to seal sheets before tiling interior wet areas. Leave to dry before tiling in the usual way.

          I used the above process to create mosaic ‘cushions’ atop a couple of log seats four years ago. Been outside in all weather, -4 to 48 degrees centigrade, scorching sun, pelting hailstones. Tiled seats as good as new. Logs not looking as flash.

          Reply
  2. Carolyn Adler

    Would several coats of varnish/polyurethane on all sides of the marine plywood make a difference? I am trying that As an experiment for a mosaics that will be under a 3 foots overhang, on the outside of a house. So far it seems that the tiles are sticking to it with the thin set. Have you ever tried that?

    Reply
    1. Joe Moorman Post author

      I’m sure it would work for a while, but eventually ambient humidity would take its toll.

      Reply
  3. Roberta

    Great article, thank you.
    Why does the foam core of backer board need to be meshed and thinnest? If the piece is going inside, or in a frame, or tiled along the edge, doesn’t glue just hold on the tiles? Is it for outdoors when it has to be covered?

    Reply
  4. Marilyn Keating

    I’ve successfully used any exterior sheathing plywood including osb for years in the northeast. If it’s prepped like a stucco house it’s fine. This means a water barrier like tyvek or tarpaper followed by stucco lath and a stucco rough coat. I live in a hundred year old stucco house and I expect my work to last just as long. At this point in time I have 20 year old pieces that are as stable as when I made them.

    Marilyn Keating

    Reply
    1. Joe Moorman Post author

      Hi Marilyn,

      I am sure that you are correct technically, but I suspect that would be problematic advice for the general public making small mosaics on plaques, especially those living in damper climates where stucco construction isn’t common. If they have one flawed place in the mosaic, then that “Achilles heal” it would be fatal because many of those mosaics would be sitting with their bottoms or edge on the ground and subjected to standing water during a rainstorm. Experience has taught me that whatever recommendations we make online have to be robust enough to work in different climates and to work when they aren’t well executed. Thanks for the feedback and tip. I am sure there are people and Mexico and California who are in complete agreement with you!

      Reply
  5. mariann mawcinitt

    Your advice is so welcome and I thank you for saving me time and misery.
    I look forward to every one of your emails. Thank you so very much.
    M. Mawcinitt

    Reply
    1. yvonne m moses

      Good information… I have a 4′ marine grade plywood that I am using for a mosaic outdoor table… Do you recommend not using it at all or will it be ok to put the concrete board on top? If I end up not using the plywood and just go with the concrete board, what thickness do you recommend I use since it’ll be a 4′ piece? I will be attaching it to the top of a wine barrel, I want to make sure that it will be strong enough.

      Reply

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