How good is premixed thinset?
Premixed thinset is nothing more than organic adhesive (mastic) with a fine sand mixed in to give it some bulk. For wall applications where mastic is appropriate, it’s fine, although I don’t see any advantage over traditional mastic. But for the use it was intended, that being replacing portland cement based latex modified thinset, it’s an extremely BAD idea, for several reasons. First, ALL mastics are formulated to be used in very thin applications. The thicker it’s used, the longer it takes to dry, and for some of the heavier notches that are used in flooring installations, it never completely dries. I personally know of one case where premixed thinset was used on a floor in April, and the following November, it was STILL soft. ANY kind of pliability will cause flex in the tile, which will cause the tile floor to fail, allowing either tile, grout, or both to crack. Secondly, even if it’s used in a thickness that WILL allow it to dry, all it takes is a little moisture to cause problem
Premixed thinset is an organic adhesive (mastic) with a fine sand mixed in to give it some bulk. For wall applications where mastic is appropriate, it’s ok, I don’t see any advantage over traditional mastic, But for the use it was intended, that being to replace portland cement based latex modified thinset, it’s a bad idea, for several reasons. First, all mastics are formulated to be used in very thin applications. The thicker it’s used, the longer it takes to dry, and for some of the heavier notches that are used in flooring installations, it will never completely dry. Any kind of pliability will cause flex in the tile, which will cause the tile floor to fail, allowing either tile or grout to become cracked. Secondly, even if it’s used in a thickness that will allow it to dry, all it takes is a little moisture in the air to cause problems. All mastics are water based, and any moisture will allow the mastic or premixed thinset to re-emulsify which causes failure in the floor.
