Help preparing an anhydrite surface

[sanorton]

Hi

I'm about to tile an anhydrite screed floor (60m2) with 60x60 1mm porcelain tiles.

I've just been reading on here that i need to sand the surface and prime it with Primer G before I tile.

So, a couple of questions:

1) How do I sand the floor? What with etc?

2) Where do I get the Primer G from.

I'm due to start tiling on Saturday, so and quick advice would be appreciated.

Cheers

Steve

[aph257]

Hi Steve,
Firstly, how long has the screed been down? you can not tile before it has dried. (it takes dry on avg 1 day for each 1mm below 40mm plus 2 days for each 1mm deeper than that) e.g. 50mm thick will take 60 days,
Sand it with any electric belt or orbital sander. hoover up all the dust prior to priming.
you can get primer G from Tile Giant or good tile shops. dilute as per instructions on container. nom 1:4 At least two coats with a roller, each coat at 90 degrees to other.

[sanorton]

Hi Paul

I cant remember how long its been down. I think a couple of months, but will check tonight.

What grit paper. 80 or something like that??

How much do I need to take off?? Sounds like a right job. Not something I’d planned for, but I need it to be right.

The priming sounds ok as long as I can get some in time.

I also have wet underfloor heating in it. Anything else I need to know?? I’ve read I need to turn it on gradually no less than 48 hours after tiling.

Thanks

[aph257]

Hi Steve,
there a couple of ways to check if it is dry, use a hygrometer, who has one of these!!, or the old plastic bag method, tape the bag down for couple of days, then take up, if it leaves damp patch then its not dried out enough.
You'll only need to texture the surface of the screed so 80 grit should be fine, just take the surface layer off. it shouldn't be arduous.
one other point, leave primer to dry between coats.

[Ajax123]

STR sander or floor scarifier from Speedy hire with 60 grit paper. If it is low laitance formulation (no skin on the surface) you will end up with a hard dense surface which impedes the penetration of the primer so make sure the primer soaks in properly. If it is the old fashioned stuff and has a soft friable skin on top it will end up looking much more like sand cement in texture but very light in colour. This will allow the primer to soak in easier.

You MUST make sure the underfloor heating has been comissioned and run. If you don't do this your floor WILL fail.

I would always recomend Gypsum based adhesive where possible as this is fully compatible with the screed. These are proprietary products and do not conform to the British Standard which unfortunately only covers Cement based adhesives. However where they have been used I am unaware of any failures.

If you need some product details PM me and I will send a web link for you to get the stuff from. If you use a cement based adhesive I would always recomend a water dispersible epoxy primer in preference to acrylic as this gives a much more robust barrier to sulphates which attack the cement causing ettringite reaction. If everything is perfect an acrylic will work but I have seen lots of failures with these usually down to poor application or poor preparation. Again I can PM you some details for various primers.

[aph257]

STR sander or floor scarifier from Speedy hire with 60 grit paper. If it is low laitance formulation (no skin on the surface) you will end up with a hard dense surface which impedes the penetration of the primer so make sure the primer soaks in properly. If it is the old fashioned stuff and has a soft friable skin on top it will end up looking much more like sand cement in texture but very light in colour. This will allow the primer to soak in easier.

You MUST make sure the underfloor heating has been comissioned and run. If you don't do this your floor WILL fail.



I would always recomend Gypsum based adhesive where possible as this is fully compatible with the screed. These are proprietary products and do not conform to the British Standard which unfortunately only covers Cement based adhesives. However where they have been used I am unaware of any failures.

If you need some product details PM me and I will send a web link for you to get the stuff from. If you use a cement based adhesive I would always recomend a water dispersible epoxy primer in preference to acrylic as this gives a much more robust barrier to sulphates which attack the cement causing ettringite reaction. If everything is perfect an acrylic will work but I have seen lots of failures with these usually down to poor application or poor preparation. Again I can PM you some details for various primers.

Could you PM me the gypsum web link as well.
Thanks

[Phil Hobson]

Some quality advice from Paul and Alan there

[deanotile]

can you pm me the primers m8

[Dave]

I think you need to make sure it is dry before you go anywhere near it.. In ideal conditions it will take the time above and even quicker if forced dry with in screed UFH..

You will need the use of a Hair hygrometer to test the residual moisture left in the screed if any.. Alan above is your man on these screeds.. but if not dry enough then there is a big risk it WILL fail.

[tommyzooom]

And your heating should be left off for 14 days after grouting

[sanorton]

The screed was done on 04/08/10 and is 65mm.

Does anyone think if i turn the U/F heating on now, would I be ok to start tiling by the weekend?

[Dave]

You need to check the moisture readings ..

[Ajax123]

The screed was done on 04/08/10 and is 65mm.

Does anyone think if i turn the U/F heating on now, would I be ok to start tiling by the weekend?

More chance of platting fog I'm afraid. The commisioning of the underfloor heating is a minimum 10 day process but in reality if you want to use this to force dry the screed it would be three weeks ish. start it off at 20 degrees C (or no more than 10 degrees above ambient) leave it there for 3 days then increase it by 5 degrees per day up to it's maximum flow temperature or 55 degrees whichever is the higher. Assume the maximum temperature is 40 degrees which would be common this is 3 days plus 4 days so 7 days. leave it at the maximum temperature for 7 minimum days whilst ventillating the screed (ventillation is extremely important) then reduce it by 5 degrees per day down to 20 then switch it off for 3 days before doing a moisture test - minimum 21 days. At 65mm you will probably need to leave it running for a few extra days as it is really a little too deep. 50mm would ave been plenty based on a 15mm UFH pipe.

Hope this helps.