i mix quite a bit nowadays,depending where the job is i tend to use different suppliers,i use mapei grout more nowadays(except white) and still use toppfix an awful lot
yes i've mixed different adhesives with different grouts but i have to admit, i am trying to keep consistant lately so my guarantees regarding mapei have the required backing.
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I mix quite a bit depending on the purpose of the adhesive and the shade of the grout - ie depending on if I need a light or dark grey, fine or full bodied grout. Same with selecting the shade of ivory etc. Most brands seem to differ in shades slightly, which is good for situations where you want to tailor the grout to an individual sitation.