Tiling bathroom floor.

I am about to tile my bathroom floor. The base is 12mm ply. I will be using dunlop 2 part rapid set flex.
Question 1.
Should i use 3mm or 5mm spacing?
question 2.
Should i seal the ply with the Dunlop Universal Bonding Agent prior to tiling?
Question 3.
Should i butter rear of tiles when tiling?
Thanks

Question 1.
Should i use 3mm or 5mm spacing?

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It entirely depends on your choice.

question 2.
Should i seal the ply with the Dunlop Universal Bonding Agent prior to tiling?

Click to expand...

If the floor is ply then you shouldn't need to although some people do to prime ply to stop colour seepage from the floor through the grout.

Question 3.
Should i butter rear of tiles when tiling?
Thanks

Click to expand...

I normally skim over the back of the tiles with adhesive using the flat edge of the trowel to fill the voids then lay as usual with adhesive on the floor (10mm notched)

Thanks, so 5mm spacing is not a must for floor tiling then.
With the dunlop 2 part set, would a normal flexible grout be ok?