Repairing small cracks in Concrete
Clean out the crack with a wire brush to remove dirt and loose stones. Wash the crack with a concrete cleaner. Get commercially available nonacid concrete cleaner made by the same company that makes your patching compound. It is safer and more convenient than the large of muriatic acid masons might use.
Let the surface dry. If the crack is deep and it is in a floor, fill it partially with sand, leaving an opening about 1/4 to 1/2 inch deep. Pour in the crack sealer until it forms a layer 1/4 inch deep. Let it dry overnight and then apply another layer. Repeat until the surface is flush with the floor. Do not overfill, apply enough patch material to bring the surface flush with the floor. If using caulk, smooth with a metal putty knife.
Fixing Large Cracks
With a hammer drill, reshape the crack so the bottom is wider that the top. This helps hold the patch in place. Chisel out any weak or crumbling spots too. If the crack is deeper than 1/2 inch, paint it with a bonding adhesive made by the company that manufactures the patch you are using. Let glue dry thoroughly before applying the patch.
On cracks up to about 1/2 inch deep, Trowel in a vinyl concrete patch. Mix according to the directions on the bag or pail, and trowel a 1/4 inch layer in the the crack. If more layers are necessary, then let the patch dry for several days before applying a new layer. Trowel the final layer flush with the surface and smooth out.
On deeper cracks, mix some sand mix according to the direction on the bag. Trowel it into the cracks, filling it flush with the surface. First, the patch on both deep and shallow cracks will have a watery sheen. When the sheen dries off, se a wooden float to smooth the surface and give it a texture that matches the rest of the concrete. If the existing concrete is very smooth, then smooth the surface with a metal finishing trowel.