Organic mastics are purchased premixed and should be stirred before application. Other thin-set adhesives consist of a combination of liquid and dry materials that must be mixed at the job site. Ratios of liquid to dry ingredients vary depending on the product but will be listed on the label. Do not mix more adhesive than you will be able to spread and cover with tile in about one hour. If adhesive begins to harden in the mixing bucket while you are working, it may have begun to cure. Do not attempt to thin it with water or other liquid as this may affect the bond; discard it and mix a new bath.
Measure liquid ingredients into a mixing bucket first, then slowly add measured amount of dry ingredients. Stir by hand with a plastic or wood paddle or with special electric mortar-mixing paddles, which can be rented at most equipment rental agencies. Whether mixing by hand or machine, be sure to keep paddles submerged. Lifting them out introduces air into the adhesive, lessening its bonding ability. Adhesive is thoroughly mixed when all dry ingredients are incorporated and it falls off, but does not run off, the mixing stick. Let mixed adhesive rest for about 10 minutes before applying.
Applying adhesive - Adhesive is applied in two steps: First it is spread, then combed. Use a notched trowel for both steps. Most adhesive applications require using a small notched trowel, but the exact size depends on the weight of your tile and the type of adhesive used. Recommendations are listed on product labels.
Apply adhesive to an area small enough to be covered with tiles during the "open time" of the adhesive. Open time, usually listed on the adhesive label, is the amount of time you have to set tiles into spread adhesive. Begin by working in a one to three foot square section. As you become more proficient, the size of the working area can be increased.
Stir rested adhesive once or twice. Scoop a small amount onto flat surface of trowel. Hold trowel with the smooth surface at an angle of about 30 degrees relative to the surface to be tiled. Press adhesive firmly onto surface, using a sweeping motion and making sure all areas are equally covered and no air bubbles have formed. Be careful not to cover reference lines. Turn the trowel so that notched edge is held at an angle of about 45 degrees and comb the spread adhesive. Comb in two passes to make a crosshatch pattern.