A levelling compound is used to treat floors that are naturally uneven in order to provide a straight surface for laying many types of tile. Most levelling compounds are sold in bags of powder that are then mixed and added to floors as needed. It is a common step in home renovation and remodelling, and many types of levelling compound are made for use on a wide variety of materials.
If you can avoid it, you should probably not use a levelling compound on a vinyl floor. Levelling mixes are designed to make smooth surfaces and are most often used on concrete or wood. Concrete often has scratches, holes, marks, and drips that create a very uneven surface that cannot be used to lay tile, while wood boards can warp over time, creating hills and valleys instead of the straight, smooth needed for proper tile insulation. Levelling compounds cover up these flaws with a putty mixture that provides a good base on which to build the tile. In construction projects, levelling compound is most often laid down on the subfloor before the floor is even set.
To properly install a new tile surface, you should remove the existing vinyl tile, grind down as much of the old tile adhesive as possible, and then apply the levelling compound to smooth out the surface for the new tile. It can be confusing because many levelling compounds are marked as usable on a variety of surfaces, including linoleum and vinyl. Just because the product can be used on vinyl does not mean it should be, and trying to bond one layer of tile over an existing layer of tile can cause problems, since the top layer of tile has to depend on the structural integrity and bond of the lower layer for its strength.
Using a levelling compound
Regardless of surface, you should try to use a primer before applying the levelling compound. A primer is a type of latex paint that will allow the compound to settle more easily and bond more purely with floor when it is dried. When you use a levelling compound, make sure you follow its mixing instructions precisely, mixing in a large bucket. If you have a power mixer or a drill mixer, use it. Try to aim for a thick, milkshake consistency before you begin to use it. Begin by pouring the mixture on the lowest places on the floor and wait for it to settle a little before spreading it at the edges with a trowel. You will probably only have ten to fifteen minutes before the compound settles, so speed is important.