Priming WBP before tiling floor

[JohnArnoldBrown]

I am tiling a small bathroom floor (when I get round to it!). I'm screwing down 12mm WBP on top of the existing floor boards, but I keep finding contradictory info about priming the WBP, some things I have read indicate that the underside and edges should be primed (with what?) and some sources recommend priming the top as well with dilute PVA.
I'm using Weber SP rapid adhesive, if that makes any difference...

I would be grateful for a definitive answer.

 

[gcol]

Personally, I wouldn't bother sealing the underside or edges.
Follow the instructions on the adhesive - if it says to use a primer on ply then use it. If it doesn't mention tiling on wood then you've the wrong adhesive (you absolutely NEED flexible adhesive). It it says you can tile onto wood but doesn't mention a primer - don't use one.

 

[JohnArnoldBrown]

The adhesive(which is flexible) says "new boards should be primed on the edges and underside" but doesn't say with what.
(By which I mean that the writing on the packet says that, the adhesive hasn't said anything yet).

 

[gcol]

Ok, basically they're covering themselves in case you use their product on a floor that is susceptible to high changes in moisture. In most homes this isn't a problem. Either that or the manufacturers don't trust their product and they're using that as a get out clause. (If you don't do it, there's no come back on the manufacturer if the tiles fail.) If you're bothered (and I wouldn't be) then give the underside and edges a coat of wood preservative (like you would use on a fence).
As I mentioned, I wouldn't bother and especially so because you're using the ply for over-boarding.
Do the instructions not mention any primer or preparation for the top side of the ply then?

 

[JohnArnoldBrown]

Do the instructions not mention any primer or preparation for the top side of the ply then?

No, they don't.
Thanks for the advice.

 

[JohnArnoldBrown]

By the way, I've read your sticky on the subject, but does the use of a 20mm notched trowel mean that the adhesive ends up being 10mm thick?

The more I contemplate this job the more scared I get!
I'm now reading that porcelain tiles are really hard to cut and drill. I have a cheap diamond-blade wet saw that goes through ceramic easily, and I only need two holes to screw the WC pedestal back down. What do you think? Have I bitten off more than I can chew?

 

[gcol]

No it won't be 10mm thick. This is because there's space between the notches. You'll end up with an adhesive bed of around 4 or 5mm.

Don't be soft - have a go, think of the satisfaction of doing it yourself.
Porcelain will cut with a wet diamond saw but it'll cut slower (you'd be better using a score and snap type cutter.) Drilling porcelain is where it'll be awkward - ideally you want a diamond drill but as you are only drilling 2 holes you can get away with a decent masonary drill. You do need to keep it cool though - what I suggest is making a small wall around the hole you're drilling with wet paper tissue then pouring some water into it so that you have a contained water supply that will keep the drill from overheating and losing it's edge - then drill the hole quite slowly.

 

[JohnArnoldBrown]

Just for the benefit of any other "soft" people out there, who might have read off-putting stuff about porcelain tiles....
I hired a Rubi TS-50 tile cutter, the first cut wasn't so hot, I don't think I was pressing hard enough , but the remaining 16 cuts were clean as a whistle.
To drill the holes, I bought a couple of 6mm tile drill bits, used a piece of masking tape to stop the bit wandering and made a well out of Blu-tak which I filled with water. Followed up with an 8mm standard masonry bit.
Fiddly bits round the doorway and radiator pipes were done with a cheap diamond wet saw.

Many thanks to gcol for all the useful advice and encouragement.

 

[gcol]

Good man, I'm proud of ya!
Rubi cutters are excellent (I have one myself ). What did they charge you for rental?

 

[JohnArnoldBrown]

I picked it up on Friday afternoon and returned it on Monday morning, and they charged me about £17, I can't remember the exact amount, but I remember getting change out of a twenty.

 

[gcol]

That's not too bad. Certainly better than buying a cheap one and bustin a load of tiles.

 

[JohnArnoldBrown]

Certainly better than buying a cheap one and bustin a load of tiles.

Or wasting hours getting sprayed with water from a wet saw!
By the way, for anyone screwing ply down over existing floorboards, I'd strongly recommend getting something like this: http://www.dm-tools.co.uk/product.php/section//sn/LNL90583
Certainly saves a lot of time!