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Posted: Mon Mar 21, 2011 9:49 pm
by fridge
A supplier has suggested that I seal the underside of a sandstone modak slab (25mm) with PVA just prior to laying the slab. Idea is to prevent moisture coming up through the ground into the stone.
Has anyone heard of this, or have an opinion on whether this is worth doing?
Intention is to seal the top once laid, but not sure if this PVA seal to the underside is worthwhile.
Any comments welcomed...
Posted: Mon Mar 21, 2011 10:00 pm
by rab1
i`m not a pro but srb is highly recommended by the pros, pva is watter based and will deteriorate over time. 
Posted: Mon Mar 21, 2011 10:20 pm
by GB_Groundworks
pva is water soluble.......how do they figure thats going to stop moisture, sbr is a bond bridge makes things stick better.
a full mortar bed and sbr in the mix should suffice
Posted: Tue Mar 22, 2011 8:45 am
by London Stone Paving
That does not make any sense. Are you sure that your supplier does not mean to stop cement residue leaching through (this is common with sawn sandstone). I dont know whether or not sealing the underside with a pva would prevent this problem, but anyhow its usually only a temporary problem that fades with time.
Posted: Tue Mar 22, 2011 7:09 pm
by cookiewales
fridge wrote:A supplier has suggested that I seal the underside of a sandstone modak slab (25mm) with PVA just prior to laying the slab. Idea is to prevent moisture coming up through the ground into the stone.
Has anyone heard of this, or have an opinion on whether this is worth doing?
Intention is to seal the top once laid, but not sure if this PVA seal to the underside is worthwhile.
Any comments welcomed...
as giles says sbr in mix semi dry full bed.modak is the hardest stone of them all it had diamond blade companys scratching there heads for a long time i have never had problems with leaching good luck :;):
Posted: Tue Mar 22, 2011 9:54 pm
by GB_Groundworks
just slightly off topic is there or has the gaffer got a table indicating the hardness of the imported sandstones etc? like from fossil mint to modak etc?
infact going to message the gaffer now.
Posted: Fri Mar 25, 2011 9:48 am
by London Stone Paving
Modak is a very hard stone. The grey Indian sandstone (kandla grey) is even harder than modak.
Apart from fossil mint all the Indian sandstones are very hard. In terms of a table Giles what do you need?
Physical data or just a list in decending order?
Posted: Sun Mar 27, 2011 10:24 pm
by fridge
Thanks all for the responses. Concensus seems unanimous - no need to seal underside. Have also spoken to a couple of landscaping builders who also say no need to do this.
Posted: Mon Mar 28, 2011 12:20 pm
by simeonronacrete
Be aware that PVA tends to reemulsify in contact with water so whatever it first may have been applied to do, it'll no longer be doing it.
Better to use Ronafix with cement in a primer/bond coat, and Ronafix in the mortar to give adhesion, strength, waterproof and frost resistance.
See
Ronafix for Bedding Mortar by Ronacrete