Working with 3/4" glass mosaic tiles for the first time - look great, but a very different process than I'm used to with ceramic tiles of normal size.

Tiled a section of cement floor following the glass tile instructions (rake with 1/4 x 3/16 V notch, then trowel flat, then place tile and beat into place) which worked pretty much fine (one loose tile from a few hundred, and several places where I must not have gotten the raking and smoothing even enough so I had to go over with a grout saw to remove excess thinset before grouting.)

Moved on to doing a section of wall that's covered with 1/2" hardibacker. Only did about 3 square feet as I figured it was going to be trickier working on the wall than the floor. Got the thinset on the wall successfully, raked and smooth, tile (in sheets with a gummed paper on the face on it and not falling down, moved to beating it in and I could hear it rattling - much of the mortar wasn't taking the tile, though enough had for the sheets to be on the wall (Mapei glass tile mortar, supposedly 2 hour pot life, 30 minute open time and conditions were not particularly hot or dry.)

I ended up stripping the tiles and scraping off the mortar. As far as I recall the instructions said nothing about pre-wetting the substrate (I understand Hardibacker to be what they call CBU), but it certainly appeared that the Hardibacker had dried it out to a point of uselessness in a very short period (which the concrete floor did not, but of course the floor needed to be mopped clean before tiling and the walls did not.) I'm wondering if I should pre-wet/dampen the hardibacker before applying thinset.

I basically wasted a few hours and a lot of mortar to re-set that one 3/4" square tile in the floor that had been loose, as opposed to getting started on the wall section...

Back-buttering these tiny tiles seems like it would result in a huge amount of cleanup for grouting if laid on with a trowel, and would be incredibly tedious dotting on the back of each one (225 tiles per sheet.)

Similar question but not exactly this (mesh rather than paper face and more about trowel technique)

1 Answer 1


The actual HardiBacker installation instructions (page 5) for tile do say to wipe the surface clean "with a damp sponge":

Wipe the surface of HardieBacker® cement board clean with a damp sponge just prior to adding mastic or mortar.

screenshot of HardieBacker cement board installation instructions for tile

This 2006 forum discussion also seems to anecdotally back up that wetting HardieBacker first is a known "must do":

Hardi is thirsty, thirsty, thirsty. It tends to suck moisture out of thinset NOW and makes working time difficult. Kinda like laying wallpaper on un-sized plaster walls.


Dry backerboard will suck water out of mortar like a sponge. If too much is removed the final bond becomes very weak.

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