Tag Archives: thinset mortar

Thinset On Plywood Mosaic Backer?

Thinset mortar can be used on a plywood mosaic backer provided the plywood has been sealed with a white PVA adhesive such as Weldbond. Otherwise, the plywood can suck the moisture out of the thinset before it has a chance to harden leaving it soft and crumbly. It is important to seal the plywood at least a day or two before you mount the mosaic with thinset because the Weldbond needs to be thoroughly dry and cured. For sealing purposes, Weldbond can be painted on with a brush or spread with a trowel. Many mosaic artists dilute the glue slightly or even up to 1:1 with water to make it easier to spread. Note that you shouldn’t seal your plywood with paint or sealers chosen at random because thinset will not bond to some of the oil-based and silicone-based products.

Some artists seal their plywood backers with Weldbond before mosaicing even if they are going to use that same Weldbond to mount the tiles. Why? They want to make sure that the entire surface of the plywood has been sealed so that it doesn’t suck the moisture out of the GROUT when it cures. I never worry about sealing the face of my plywood because I am sure that I will get enough Weldbond spread around when I mount my tile, and so pre-sealing seems like an unnecessary extra step to me.

Sealing VS Sealing

A note of clarification: In this article, I am talking about pre-sealing the face of the plywood with Weldbond or some other PVA adhesive. This is a different step from sealing the finished mosaic a few days after grouting with a tile and grout sealer or “sealing” the edges and back of the plywood with paint or varnish to prevent warping by moisture.

When You Should and Should NOT Use Plywood

Plywood can warp and delaminate over time merely from the humidity in the air, and so you should never use plywood as a backer for outdoor mosaic or a mosaic in a damp location. However, plywood makes a great backer for dry indoor mosaics and not just for its light weight, which in itself is a significant advantage over concrete backer board. Unlike concrete backer board, the edges of plywood can be stained or painted, and they do not shed crumbs of concrete and sand. In my opinion, concrete backer board should only be used inside walls and floors and other places where its crumbly edges are covered up; it is not a backer for moveable mosaic plaques.

Use Cabinet-Grade Plywood

Note that you should spend the extra money and buy cabinet-grade plywood (in 1/2 inch thickness) instead of the ordinary plywood used for sheathing in construction. The cabinet-grade stuff comes with a sanded finish and has no knots or internal voids. It is also more resistant to warping, and the edges of cut pieces are cleaner than those cut from regular plywood mainly due to the lack of voids in the internal plies. With all of these advantages, the cabinet-grade plywood is only about 15% to 25% more than regular plywood, and in real terms it is actually the same price: some of the regular plywood may need to be scrapped due to internal voids, and it will require more work to make the edges presentable. The edges of a cut piece of cabinet grade plywood can be left plain or painted. The edges of a cut piece of regular plywood will have voids and require putty, sanding and paint if not a frame to look presentable.

Why Use Thinset on Plywood?

If plywood can only be used for dry indoor mosaics, then why would someone need to use thinset mortar on a piece of plywood anyway? The simple answer is there isn’t any reason to do so (IF you are using regular flat tile). With flat glass tile, you simply glue the tile on with Weldbond or some other PVA adhesive, let it dry for a day or so and then grout it by rubbing wet grout into the gaps and wiping away the excess.

However, some people (including myself) like to use rounded and irregularly-shaped found objects in our mosaics, and these are not easily grouted. Instead of the glue-then-grout method, we like to press our objects into a bed of mortar so that the excess mortar presses up slightly around the object and skip grouting entirely. It is also necessary to avoid grouting if your found objects are naturally porous (seashells, bones, unpolished stones) because grout will stain these materials.

solstice-door mosaic detail

The face of “Solstice Door” mosaic stele was made from a lifetime’s collection of found artifacts embedded in thinset mortar. Note that the stele is a reinforced concrete monument. Plywood would not support the weight of these materials.

How To Use Found Objects In Mosaic Art

Making and grouting a mosaic from ordinary flat tile is relatively straight forward. However, adding three-dimensional objects such as porcelain figurines or even simple rounded stones can complicate the grouting process. Instead of using a grout float or damp sponge to wipe away the excess grout, you may need to use a gloved hand as we often do, and you may have “excavate” some objects using brushes or dental picks if they have very complex shapes with nooks and crevices. An old toothbrush can be useful for cleaning wet grout from crevices in objects, but note that brushes of all type remove grout very aggressively, so take care that the toothbrush doesn’t gouge out any grout from where it needs to be.

If your mosaic is mostly dimensional objects, then the amount of labor required to remove excess wet grout may be excessive, and the easiest way to solve the problem is to avoid the grouting process entirely. Also, unsealed porous objects such as seashells cannot be grouted without staining them with the grout.

For these situations, it is better to press the objects into thinset mortar instead of the usual glue-the-grout method used for making indoor mosaics.

Thinset is a type of concrete, so if you spread it on slightly thicker, then a little bit will press up around the sides of the objects and serve as grout between the tesserae (“tile”). The key is to not spread so much thinset so that the object sinks in too deeply or too much thinset squeezes up and around the object.

Before you start pressing your childhood treasures and precious souvenirs into the sticky wet concrete that is thinset, you will want to practice for a few minutes with some ordinary rocks on piece of scrap. That way you can figure out how thickly to spread the thinset to avoid problems and practice keeping your hands clean while working with wet concrete. Otherwise your fingertips are likely to get wet concrete on objects that aren’t easily cleaned.

Your quick experiment on a piece of scrap should use more than one size of rock and include pressing objects close together. That way you can decide how closely you want to space your found objects in your actual mosaic and see how small objects have the potential to be buried in thinset pressed out by large objects nearby.

Keep in mind that small experiments like this do not require any extra time. Whatever time that is required to do them is almost always made up for in the time they save you on your project.


How To Build Up Low Areas With Thinset Mortar

Low areas on a surface can be built up with thinset mortar mixed with an additional aggregate of very coarse sand a few days before tile is laid. This should be be done if you want to put a mosaic insert of thin glass tile (1/8 inch) between thicker architectural tiles like 3/8 inch stone and ceramic and still have a flush surface.

A Separate Step Is Best.

This must be done before you mosaic, preferably a few days before or even earlier. Remember, thinset shrinks or thins as it cures. You should not try to tile and build up areas at the same time.

How to Minimize Thinning During Curing

The idea is to add coarse sand with the fine stuff sifted out. (This is in addition to the fine sand aready in the thinset.) The best aggregate is the smallest grade of crushed stone. You want the largest piece size that is practical. For example, if you are building a layer 3/8 inch thick, you might not want a grit larger than 1/16 inch. If the grit is too large, you might have trouble making a smooth level surface that doesn’t have pieces of grit sticking up too high.

Make sure you save about 20% of your thinset batch free from coarse sand. You will need some plain thinset for wetting surfaces before applying the thinset with aggregate added. Scoop this into a separate container after you mix up your thinset.

How To Sift Sand Without a Mesh Sieve

Of course your sand has to have some coarse stuff in it for this to work. Bags of sand from the concrete aisle of the building material store won’t work for this because it is already sieved to be fairly small and uniform.

Colander or Two Buckets

If the sand is coarse enough, you may be able to wash it in a colander. If it is smaller than the holes of your colander, then you can use two buckets.

Do It Outside With a Garden Hose

In either case, do it outside with a garden hose somewhere water and sand can slosh. Sand should never go down drains.

2-Bucket Method

Put the sand in a 2-gallon plastic bucket. Fill the bucket 1/3 the way up with sand. Fill the rest of the way up with water. Swirl the sand with with a large serving spoon (Use a one-piece spoon. Don’t use welded ones. The scoop tends to break off the rod eventually.)

Allow the sand to start to settle, but pour it off when a sand slurry is still spinning around in the bucket.

Pour it off in a neat flowing motion that doesn’t jerk or jolt the bucket or otherwise prevent heavier particles from settling out. Don’t pour off the sand in the bottom.

A 5-gallon bucket is a good size for the bucket being poured into. Allow the 5-gallon bucket to settle for a few minutes and pour off the water in it.

Add more water to the sand still in the original 2-gallon bucket and repeat the operation a few more times or as many times as needed.

Start With Mostly Coarse Sand

It really helps if you start with sand taken from a place where the sand is already the coarsest. Look at the pile at the garden center or creek bed and pay attention to where the water has washed the smaller pieces away leaving mostly the good stuff.

Pick out random rocks and pieces that are too large.

Mixing In The Coarse Sand

The sand needs to be dry before you mix it into the thinset, else you are adding water to your thinset.

Add no more than 50% coarse sand, the coarser the better. If your sand is too fine, it will overload the thinset because the small stuff has a lot more surface area to wet. Make sure you don’t add more sand than the thinset can coat. Add some sand and mix it in before mixing it all in. Stop as the mixture becomes saturated. The coarse grains need to look like something breaded for frying.

You can also so this with small pea gravel to make 3-D concrete figures for mosaic sculpture.

Applying To The Surface

Wet the surface with thinset before troweling on the thinset/coarse sand aggregate that you mixed up.

Keep in mind that it will still contract some if the low spot has any depth to it. The coarser the sand you use, the less it will contract. It is possible to make the bulk contraction small enough
to not be noticeable.