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.
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.