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I'm currently placing mosaic on a concrete staircase. When I push the tiles into the thinset often the thinset is spilling through the gap. I'm removing the excess with a brush which is annoying because it moves some tiles from their place.

I'm wondering if that's a symptom of a wrong technique. Thinset too soft? Too much thinset? I'm using a 4mm v-trowel.

Thanks, Gerard.

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up vote 3 down vote accepted

Your thinset should be like peanut butter. If it is too thin then you could have possible issues with mosaics and flooring. Basically you could push down to bare floor or close to it.

A 4mm trowel is perfect size for most mosaics. I think that your issue is that either you aren't combing the area well enough after dropping down your thinset or possibly you are pushing down to hard.

The installation method for any tile but especially a mosaic is to comb the area in a direction, sit the tile on the area, then gently press the tile into the thinset perpendicular to the thinset streaks back and forth. If you were to just press straight down hard you would have a lot of thinset in between your tiles and as mentioned before not enough behind the tiles.

Also you will always have a little spillover. You don't have to clean out all of the thinset in between the tiles. Your grout will bind to the thinset fine so you just need enough room for the thinset to not show.

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Thanks for the advice. Thinset of 'Peanut butter consistence' Good tip. Mine was a bit softer. I'll try for the next round of steps. – maasg May 11 '14 at 6:52
@maasg - make sure you are not pushing down too. You should only have a downward force when moving the tiles back and forth perpendicular to your thinset lines. – DMoore May 11 '14 at 14:22
I've been tapping down the mosaic with a rubber hammer and a wood block like I've seen in several youtube clips. I'll try a gentler push as you mention. – maasg May 12 '14 at 10:26
@maasg - youtube.com/watch?v=pyt2US_SSBM – DMoore May 12 '14 at 13:10
Excellent video. Will certainly try that. Thanks a lot. – maasg May 13 '14 at 10:32

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