The most commonly used microtopping is put down by trowel or squeegee. This type usually requires a minimum of two coats, and in some cases three to four coats depending on the condition of the subfloor and the desired look. The first coat — called the body or structure coat — will contain courser sand. The next application will be a finer-sanded version of the same product. Some manufacturers even have superfine mixtures for use as a finish coat.
The second type of microtopping — sometimes called a semi self-leveler — is a cross between the traditional trowel- or squeegee-applied version and a self-leveling overlay. These products have self-leveling properties but are usually applied at a maximum thickness of only 1/8 in. (vs. 1/4 to 1 in. thick for the typical self-leveling overlay). A semi self-leveling micro topping is typically applied in one lift rather than two or more separate coats, but the finishing process requires a high degree of skill during the troweling process with the ultimate goal being a polished look similar to that of hard troweled concrete. Bond or primer coats are important for several reasons when using these types of systems. If you install a microtopping on unprimed or prewet concrete, the overlay system could dry instantly. That's because the porous, unsealed concrete would absorb the moisture in the topping, causing it to flash set and jeopardizing its workability and adhesion. In lieu of a primer, some manufacturers recommend applying the micro topping to concrete that's prewetted to a saturated surface dry (SSD) condition to minimize absorption of moisture from the fresh topping. When the substrate has been totally sealed off with the proper amount of primer or is saturated surface dry, the moisture in the microtopping is not absorbed into the substrate. Instead, moisture bleeds to the surface of the topping, which facilitates troweling.
After all of the necessary surface preparation has been accomplished (profiling, crack repair, priming), the next step is to apply the material to a depth of about 1/8 in. with a gauged roller or a gauge rake set at 1/8 in. These tools help to spread the topping evenly while ensuring a uniform depth, but they aren't designed to move large volumes of material. When pouring the freshly mixed topping from the bucket onto the floor, apply it in a uniform ribbon rather than dumping it in one area. This will make it easier to spread properly. (It's best to have one person do the pouring while another person uses the gauge roller.) Try to spread the semi self-leveler in sections 2 to 4 ft. wide, making sure to avoid cold joints (distinct lines that form between sections that dry at different times).
Once applied, these systems generally require troweling, usually in two stages. The first troweling should begin within 10 minutes after you spread the microtopping. If you want to produce a hard-troweled surface similar to the look and feel of smooth, hard-troweled concrete, then a second troweling will be necessary. The timing for the second troweling varies depending on the size of the project and temperature conditions. I find that the most opportune time to second trowel is when 75 percent of the surface bleed water has evaporated, leaving behind patches of damp areas on the surface. Be sure to kneel on spiked kneeboards or wear cleated shoes when troweling to avoid leaving foot or knee prints in the fresh topping.
Remember, the warmer the temperature conditions, the faster the material will set. For this reason, try and apply your toppings in the coolest part of the day.
It is also imperative to keep any source of air from moving across the surface when you're applying the topping to avoid rapid drying — especially with a semi self-leveling product. This caution applies to both indoor and outdoor projects. On one interior flooring project, for example, I installed 1,500 sq. ft. of microtopping that went down beautifully, except in an area where I had left two doors open on either side of the building. The cross draft caused the material to skin over in that area. Fortunately, I discovered the problem in time to avert disaster.