Premix vs thinset

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Thinsets and premixed adhesives are really different animals. Thinset is a concrete based mortar and gains its strength through a reaction that occurs in the presence of moisture. Premixed adhesive sets by evaporation. If the surfaces being glued are impervious, I have actually seen premixed adhesive never set. Weeks later you can go back, pull off tile and the adhesive is still sticky and workable. Never had that happen with thinset. I would use a premixed adhesive on semi-permeable surfaces to set relatively small tiles with frequent grout lines allowing the glue to set throughout. Pre-mix is generally finer and trowels and combs to a finer consistency. This makes setting finer tiles easier. It also has a long workable pot life allowing work to proceed without additional mixing. Premixed adhesive allow you more time to clean up tile in case you get some on the surface, or leave too much in the grout line. Always read tile compatibility and intended interior / exterior, dry/moist application of pre-mix.

I always use thinset on the back of stone tile and prefer it in any larger tile application and use a latex modified thinset for moist areas (except marble and travertine which is incompatible with modified). Thinsets can be modified to be an effective moisture barrier. The workable pot life of thinset is 20 to 40 minutes and it helps to have someone doing the mixing of the next batch while you finish laying tile. It is a little more work, but I have found the result to be stronger and more predictable. Thinsets can be used outdoors, while many premixed adhesives are not suitable in moist locations.