Bringing up the age-old topic of tiling on wooden floors, I've read through
the archives and think I have everything about sussed. My only source of
confusion is the sealing of the ply under the tiles. It's in a bathroom so
I need to be absolutely certain that water isn't going to permeate the ply.

So, am I best sealing the ply with:-

a) Wickes Tiling Primer additive - at £9.99 a bottle it's not particularly
cheap.
b) 3:1 Diluted PVA OR
c) Polyurethane varnish

I'm tempted to go for c) since I would imagine it would be most waterproof
and I already have a tin of the stuff. Can anyone think of a reason NOT to
use this though?

AND...

Anyone got any experience of drilling 8mm ish holes through 10mm ceramic
floor tiles?

Ta,

Andy