Large format tiles advice

[Sludd]

I've got some large tiles from Porcelanosa to fit in a quadrant shower enclosure.

The tiles are 1000mm x 333mm and are being laid horizontally, as its a 1000mm enclosure.

The substrate is waterproof tile boards - Abacus elements brand. Taped at the joints and screw holes.

I'm wondering what size trowel is most appropriate for this size of tile? I've never used anything as large before.

And is back buttering the tiles a good idea?

Also, the walls dip a little in the middle of each wall by about 3-5 mm. I got some advice on the adhesive forum a few weeks ago (I've been waiting for 4 weeks for the tiles to arrive) to try to skim out some of this dip, rather than just lay a thicker bed of adhesive in the middle.

If I was going to do this:

1. Should I use the same adhesive as for the tiling and just let it set before fixing the tiles?
2. Do I just skim the wall where it dips and use a straight edge to true up the walls?
3. Should I key the surface of the adhesive once skimmed?
4. Will any of this reduce the adhesion of the tiles, as the substrate basically becomes previously set tile adhesive?

Apologies for lots of questions - I'd prefer to get my ducks in a row before I start rather than have a problem later.

 

[Time's Ran Out]

It’ll be fine as long as you solid bed fix the tiles.
Use a few levelling clips to keep the edges flush, 3-4 along each edge.

 

[Flintstone]

12mm trowel with a back butter and a little extra in the middle by the sounds of it

 

[Sludd]

Thanks guys - I hoped I'd be OK without skimming the walls.

I have round notched trowel I usually use for floors but will buy a 12mm as well and try a couple of tiles with each one, take one off and check for coverage.

Appreciated.