4 Year Old Gyvlon (anhydrite) preparation advice

[Algy]

Oh and the primer I was recommended by the original flow screed layer is Ardex P51 - any thoughts on that?

 

[thickbed]

If you removed 1mm of laitance, you would be down to aggregate
There is a lot of conflicting advice as to how much laitance to remove but personally I take off enough to expose the aggregate then prime with a good quality primer sticking a anti crack mat down with a gypsum based adhesive then cement based above mat to stick tiles with.

 

[Ajax123]

And how do you measure this mythical 1mm. Sorry but it's rubbish. I've heard everything from 1mm through 5mm to one adhesive rep who said you have to sand until it's too hard to sand further and if you get through to the pipes the screeder has to replace the screed.

You are simply sanding off any loose friable Laitence with loose and friable being the key phrase. You don't scratch off with a screwdriver, you don't scrape off with the corner of a paint scraper, you don't bang it with the edge of an old chisel. Once the surface is sound stop sanding. This could be as much as 1mm but as little 100 microns. It's not measurable in depth terms. The requirement to expose aggregate is a myth. The screed is porous enough to accept a suitable primer e.g p51, p121, prime plus, apd, ar and many others.

The most important elements are make sure it's dry enough and where possible use gypsum based adhesive

 

[Ajax123]

In this instance what dilution ratio (if any) would you use if you were using say, SBR
Just seems be an important part of the job which can really go t*ts up,if not done correctly.

Typically 1:1 with clean water although it depends on whose primer it is. Some are 1:3 and some are already dilute enough. If in doubt test it in a small area first. If you are using gypsum adhesive you are in my trying to reduce suction so much less primer required. The water soaking into the surface via the life structure effectively pills the polymer in as well. If using cement you need to form a chemical barrier so more primer but then it can skin over so is higher risk. Sometimes of cement is used it's better to use an epoxy primer.

 

[thickbed]

And how do you measure this mythical 1mm. Sorry but it's rubbish. I've heard everything from 1mm through 5mm to one adhesive rep who said you have to sand until it's too hard to sand further and if you get through to the pipes the screeder has to replace the screed.

You are simply sanding off any loose friable Laitence with loose and friable being the key phrase. You don't scratch off with a screwdriver, you don't scrape off with the corner of a paint scraper, you don't bang it with the edge of an old chisel. Once the surface is sound stop sanding. This could be as much as 1mm but as little 100 microns. It's not measurable in depth terms. The requirement to expose aggregate is a myth. The screed is porous enough to accept a suitable primer e.g p51, p121, prime plus, apd, ar and many others.

The most important elements are make sure it's dry enough and where possible use gypsum based adhesive

Why the need for such an arrogant response? I know you are a very well respected member from the screed industry and I wasn't trying to pick a fight! I was saying that this was my personal method of laitance removal from liquid screeds. I have invested a lot of money in sanding equipment, hoovers etc and have done a fair bit of laitance removal and this is what Ive found to be the better method.
From copper discs to sanding pads to diamond cups, Ive tried them all numerous times and honestly a 60grit sanding pad will do absolutely nothing to a screed. A copper disc will scratch it but not remove all of it and a diamond cup setup is by far the best .

 

[Ajax123]

Bad day... sorry.

 

[Algy]

Many thanks to those who have replied. It has helped. I guess it's an area both of differing experiences / opinion and maybe a number of different ways of achieving the same thing - you can see how I could get confused.

But I will do the light grinding (glad to hear 1 mm may be more than needed) and use probably P51 as the original screeder suggested, or ask carefully why the profession that I will have tile the floor, thinks another primer is better.

I'll also have a decoupling mat and see if the tiler agrees on a gypsum based adhesive for the mat and then cement based above mat to stick tiles with.

Can't do much more.

 

[thickbed]

Many thanks to those who have replied. It has helped. I guess it's an area both of differing experiences / opinion and maybe a number of different ways of achieving the same thing - you can see how I could get confused.

But I will do the light grinding (glad to hear 1 mm may be more than needed) and use probably P51 as the original screeder suggested, or ask carefully why the profession that I will have tile the floor, thinks another primer is better.

I'll also have a decoupling mat and see if the tiler agrees on a gypsum based adhesive for the mat and then cement based above mat to stick tiles with.

Can't do much more.

I am based in Oxfordshire and see you are in Berkshire, I am happy to come and take a look and give you a quote for the sanding if you like
PM me if interested
Many thanks
Mike