Grouting Mosaics with Cavities

[White Room]

If you used a larger notched trowel the addy may squeeze through and cleaning mosaic joints is no fun, a 4mm maybe better

 

[Bolter]

Once you have fixed a row of mosaics, I use my rubber grout float and gently push/run this over the mosaics to get them flat.

An off cut of tile works well also.

 

[The Legend; Phil Hobson RIP]

If you used a larger notched trowel the addy may squeeze through and cleaning mosaic joints is no fun, a 4mm maybe better

I would go for the 4mm trowel, it will help with the thickness difference. More important, it will give you better coverage on mosaic.