The first thing you need to know about outdoor and wet mosaic (pool, fountain, shower) is that thinset mortar is used instead of glue. Thinset mortar is a type of concrete with polymers added for tensile strength and enhanced adhesive properties. It contains sand and portland cement just like standard concrete and comes as a powder which is mixed with water. Hardened thinset is like concrete, only stronger and tougher (able to withstand greater impact without cracking).
I wrote a must-read page for how to use thinset mortar for detailed mosaic artwork. The thinset page contains basic handling instructions and a procedure for keeping your hands clean as you work, which is actually not as simple an issue as you would think if you haven’t ever done much work with sticky substances like construction adhesives.
I have also written a summary of basic pointers for outdoor mosaic art and an illustrated case study based on a customer’s street number project. Both of those pages contain information that can make your outdoor mosaic more durable and less labor intensive to make. (One caveat: the case study documents how our customer did his particular project, which could have been done much more efficiently by laying up the mosaic temporarily on paper, but what it lacks in efficiency it more than makes up for as an example of how much shear energy amateur artists bring to their first projects in a new medium. The sequence of photos is also instructive.)
BIG Bags of Thinset
The first thing you will notice when you go to the building material store to buy thinset is that it comes in 50-pound bags. Only 50-pound bags. As if people never had to tile the floor of a shower that was only 6 square feet. Or make mosaic art or do any number of household projects that would require nowhere near 50 pounds of anything.
For over a decade I have been hoping to find a manufacturer who could supply thinset in smaller containers, but no luck yet. I’m not sure exactly why a company the size of TEC or Laticrete wouldn’t value someone like me distributing thousands of samples of their product and paying for the privilege of doing so, but the shear lack of imagination in corporate America has never ceased to amaze me.
Imagine if you could only buy nails in 50-pound bags. Think that might kill some potential sales?
The good new is that thinset is a very useful product for home repairs, and there are ways to make the 50-pound bag more easy to use. Also, mosaic art tends to be addictive and serialized. If you make one, you will make several in all probability. Statistically, you are also likely to return to the craft repeatedly over the years.
How to Manage a 50-Pound Bag
Tip #1. Buy a 5-gallon plastic bucket with lid when you buy your bag of thinset. If you keep it sealed in the bucket where moisture and humidity can’t degrade it, the thinset should last for years and years.
Tip #2. Read and copy any instructional information from the bag before you put it into the bucket. If you forget step #2, you can always read the outside of another bag at the store or look up the manufacturer’s product information online.
Tip #3. Put the entire bag into 5-gallon bucket. Do not attempt to pour the bag into the bucket unless you want to create a dust cloud the size of Texas and coat the inside of your lungs with fortified concrete.
Tip #4. Slit the top of the bag with a box cutter and keep an old measuring cup or plastic tumbler in the bag inside the bucket for scooping out thinset as needed.
Tip #5. Scoop and mix your thinset outdoors in a location you can hose down. Or just be neat.
Tip #6. Mix your thinset up in an oversized container. For example, if you are mixing up only 3 to 5 pounds of thinset, you should still use something the size of a 2-gallon bucket. That will help contain the dust and the splatter. If you use a container that isn’t much larger than the mass you are mixing, then you will probably sling dust and mud out. Concrete is heavy and thick and takes concentrated force to mix it. That means it is more likely to be accidentally slung while being stirred.
Tip #7. Pour the measured water on top of the thinset to be mixed up. Blend it slowly and carefully at first to minimize the amount of dust being created.
If you are mosaicing on an outdoor wall of stone or masonry or a slab of concrete, then make sure the surface is clean and free from paint and any outer layers that may have been degraded by exposure to the elements.
Note that degraded surface conditions might not be as obvious as flaking paint chips. It might simply be that the outermost layer of stone or concrete is a little more crumbly than the same material underneath.
The easiest way to test for this type of superficial decay is the same method you would use to remove it if it exists. Take a wire brush of the type used for cleaning welded metal and scour the surface. If surface material comes off relatively easy, then you have some idea how much scouring you need to do.
Remember it doesn’t matter how well you mount your mosaic or seal it if you mounted it on a dirty or degraded surface. The tiles will start falling off relatively quickly.
The Right Brush
Note that the wire brushes used to clean welds have wooden handles and thicker stiffer bristles than the wire brushes used to clean barbeque pits. They are available at most hardware stores.
If you are installing a large outdoor mural, then you may want to use a circular wire brush on a power tool such as a drill or angle grinder. But, as my father used to say, “Boy, put you some long pants on and keep a good hold a this thing cause it’ll take the hide clean off you.”
Wearing the clear plastic face shields used by metal workers are also a good idea, especially if you are a novice. Gloves are not optional when using any power tool that removes material.
Plaster The Surface Level
When we mosaic a masonry wall or fireplace, we will often plaster the surface level a few days before mounting the mosaic. What do we use? Good ole thinset from our 50-pound bag.
Note that thinset gets thinner as it sets or cures (hence the name), and so you should be aware that any divots or trenches you fill with thinset (such as the seam between bricks) may start off level, but as the thinset cures, a ghost of the original divot or trench will reappear. If the holes I am trying to fill are more than half an inch deep, I will reapply a second plastering of thinset after the first coating has cured for three days.
Pre-Mount Your Tiles
Doing detailed work outdoors is much more difficult than working at your studio table, even if you are doing it on the first magical day of warm spring weather or the first refreshingly cool day of fall. Even in pleasant weather, there are ergonomic limitations that quickly come apparent when you attempt to set each tile by hand, and then there are only so many daylight hours and so many days that you can be on the job site.
The solution is to lay up your mosaic before hand. I recently wrote an illustrated post that explains how to use sticky contact paper and clear packing tape to lay out mosaic designs in advance of actually mounting them. We also sell mounting paper and mounting grids in our mosaic tools category.