a water biased primer on the pipes will have no more effect than the water in the pipes
in fact it will create a barrier between the pipe and the cements adhesive witch are more likely
to harden the pipe making them brittle over the years .
you need to go back to the drawing board and right a full spec for tile fixing
to your floors taking into account all of the above we have already talked about .
1 you need to start by under standing what affect expansion has on tiles on pipes
2 understanding that not all flexible adhesive are S1 or S2
3 how much pressure cold water should the pipes be under while being tiled and why
4 what benefit would a 1mm decoupler give to the floor
5 under standing of primers why prime why not
6 time needed to dry before commissioning
7 how to commission the floor on start up
8 understanding the effect of cement adhesive on gypsum boards
9 the need for expansion joints through tile fields of 25m2 and more with tiled floors
with under floor heating .
10 expansion allowance on cutting round door frames and architrave and all perimeters walls internal and external .
as an under floor heating firm these might not seem relevant to you .
but they are essential to tile fixer and end users .
thank you for all your answers at least you are trying to get there but you
need a full tilling spec. not a one size fits all answer for all floor coverings