I have no reason for this other than personal preference.
Thinset between mosaic tiles.
A 4mm trowel is perfect size for most mosaics.
Thinset is inexpensive and it fills in gaps and depressions.
When used to tile walls it s strength allows you to set tile and if you re good with eyeballing you can set tile without the use of spacers.
Thinset is a bonding mortar which means it is a portland cement the same as used in concrete with polymers added to make it sticky and extra strong.
For outdoor and wet mosaics thinset is recommended instead of adhesives.
Mastic is very sticky grabs fast and is quick setting.
Basically you could push down to bare floor or close to it.
Thinset is slow to set which leads to tile sag on vertical applications.
If you attempt to set wall tiles using thinset without spacers you ll end up with a bunch of tiles that have slid into each other before the thinset has had time to set resulting in a giant mess.
If it is too thin then you could have possible issues with mosaics and flooring.
When i weave my lines i like the thinset to be smooth especially where there s more open space.
Smooth thinset in between lines and tesserae.
When installing glass mosaics excess thinset will bleed through the grout lines and create a disjointed uneven tile project mess.
This is particularly important when working with glass mosaic tile which is non porous and difficult to bond securely.
Once i set my tesserae into the thinset i leave them for a bit before attempting to go in with my various implements to smooth it out.