Mosaic and uneven substrate

[yffe]

I am considering installing some white glass mosaic (5 x 5 cm on mesh) in the kitchen above the sink and have some questions about the substrate, which is a plastered brick wall from the 60s.

When I removed the old tiles, a fairly large part of the plaster came off with them (about 2 cm thick) and I have now patched it up with gypsum plaster. However, it's not completely even as there are both small imperfections, think bumps and pits of a few millimeters, and larger irregularities. Using a meter stick, it differs up to a centimeter.

What do you think, is it possible to install the mosaic directly on this, or do I need to level it with something? I would prefer to avoid using renovation plasterboard. How "forgiving" is installing mosaic?

 

[Locke]

No, then you'll be in a hell of a time - it has to be completely flat underneath otherwise the mosaic pieces point in all directions - also you'll have to wipe the fix out of the joints until you get tired...

 

[slacker]

No, then you’ll have a hell of a time - it must be completely flat underneath, otherwise the mosaic pieces will point in all directions - plus, you’ll have to clean fix from the joints until you get tired...

Locke is absolutely right - The substrate must be good for setting mosaic. Roughly sand it down and maybe do a round with regular sand filler?
5x5 can handle some minor irregularities, but if there’s edge lighting from the bench lighting, you’ll see flaws.
It can help to press the tiles into place with something like a float. (presses about 10 tiles at once)

I always claim that tile setting is only as good as the substrate, and that's true....