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Sealing walls with PVA for ceramic tiling

Sealing walls with PVA is a very much misunderstood operation. DIY Doctor gets many emails saying that PVA should not be used on walls before they are tiled. This is simply not true. The fact is that they do not understand the process of using PVA properly. PVA can be used to seal any type of porous surface,including plasterboard and if it is done properly it will give you a much much better job than working on unsealed walls. To seal a wall the PVA should be dilted at about 20 – 25% PVA to 80 – 75% water. This type of dilution allows the PVA to soak into the material with the water sealing not just the immediate surface but a little bit deeper into the material. Not only does this form a proper seal but it makes the top layer of the material stronger by binding it fully with the depth of wall immediately below the surface. To those who tell us they have been tiling for 40 years and they KNOW that PVA should never be used, we say that you may well have been tiling for 40 years but you have never taken the 5 minutes or so that it takes to understand how PVA works!

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8 comments

  1. Finally a video explaining things simply for beginners, thanks

  2. Have to say been using PVA at 50/50 for about 25 years on everything including external render between coats and as a paint primer never once has it ever failed, including my latest fully tiled (floor to ceiling) bathroom now 3-4 years old on dot 'n' dab new drywall (again used it on both brick and board) at a tile weight of 22kg per metre (plasterboard limit 25kg) including wet shower area with 3 coats PVA. I kind of think that I have fully tested the integrity of diluted PVA and although I agree 75% maybe a better formula I'm really quite ok with 50/50.

  3. I'm a DIYer and not a tradesman. Just want to say how helpful your videos are in clarifying a lot of things that I'm not sure of. You're good at explaining why things happen and the consequences if you do not do it right. Thank you.

  4. I have been fitting tiles for 36 years and using  diluted exterior PVA as a wall primer is better than no primer. its that simple. most tiles come lose because no primer was used.

  5. Unibond recommend pva as a sealant for their "Tubbed addy" 

    NO REPUTABLE cementatious based adhesive supplier will……..

  6. as a pro tiler for over 32 yrs i recommend you follow the instructions of the adhesive supplier . most DO not recomend polly vinyl alcohol (PVA) AS IT SITS ON THE SURFACE IF THE SUBSTRATE.most adhesives work by crystalizing when they set. the crystals at a microscopic level penetrate the surface of the substrate creating a fix PVA prevents this happening so the adhesive sticks to the pva and not the wall . i make a lot of money repairing tile failures by diy and ignorant professionals so if you want to make me more money please use PVA.

  7. PVA stands for polyvinyl acetate, and it is a rubbery synthetic polymer. It is commonly emulsified in water and used as glue. Many know it simply as "wood glue", or "carpenter's glue".

    Cementious materials, such as many tile adhesives and grouts, or other materials which contain cement, such as concrete, are alkaline. Simplified, that means they have a high pH.

    Alkali slowly attacks polyvinyl acetate, forming acetic acid, which has a low pH. Cement doesn't dry per se; it cures through hydration, which means it binds the water you mix it with chemically. This causes the pH of the substance to rise dramatically. Introducing an acid negates that process to some extent, preventing the cement or conrete from binding all the water it needs to harden properly.

    It is hydrolysis which gives cement and concrete products strength, and holds them together. Without this process, it would merely be the powder you started with.

    The acetic acid which is formed when cement and PVA comes into contact, either through mixing them, or "priming" with PVA, will continually free the water bound in the cement, and that will weaken the bond and/or integrity of the material. The effect is accelerated if the material is subjected to moisture, which is more or less always the case.

    PVA isn't water resistant. It becomes slightly live when exposed to moisture, and this in combination with the exposure to alkali, accelerates the forming of acetic acid. PVA which is marketed as "water resistant" or "exterior grade", has additives which makes them water resistant, but they're not alkali-resistant.

    Anhydrite, gypsum, and cement

    Anhydrite products are mainly composed of calcium sulfate, and gypsum products are mainly composed of calcium sulfate dihydrate. When anhydrite is exposed to water, it forms gypsum. In other words, it hydrates. Essentially, it cures, but not to the same extent as cement.

    Gypsum always has a proportion on anhydrite crystals left in it.

    Cement has a proportion of calcium aluminate. Calcium aluminate reacts with calcium sulfate, which is the main component of anhydrite products, and which is present in gypsum. The reaction forms hexacalcium aluminate trisulfate hydration; in other words, ettringite crystals. These expand, and force away anything which is fixed onto where they form.

    As I've previously explained, cement cures, which means it binds water through hydrolysis. That means water is always present in cement. If anhydrite is put into direct contact with cement, there will be a reaction. The reaction won't be as severe with gypsum, as it's already hydrated most of the anhydrite (the dihydrate part), but there is still some present.

    Thus, if you want to tile onto such products, you will need to separate them entirely. This is best done with a products which seals, and which is also water resistant, such as acryllic dispersions.
    Even if you use water resistant "PVA", the separation will deteriorate with time, due to the chemical reaction between the cement, which is alkaline, and the polyvinyl acetate. If the bond of the cement onto the substrate hasn't already been compromised because of that, the formation of ettringite crystals will very likely cause complete debonding.

    Rapidly curing cements may have some gypsum added when manufactured. It accelerates the curing, but does not affect the integrity of the product, because it's present in such small quantites, and during the early stages of curing.

    Final Notes

    PVA is not suitable as a primer, sealer, impregnator, or admix. The uses of PVA may be many, but they do not include anything tiling related. Use proper manufacturer approved primers and additives instead. Using PVA will likely cause liability issues when problems arise, and that is bound to cost alot more than buying proper materials to begin with.

    If you want to tile onto anhydrite or gypsum, make very sure to properly separate the substrate from the adhesive. There will likely be tears otherwise.

  8. if you do what this guy said watch your work fall of the walls pva IS A NO NO WITH MOST REPUTABLE ADDHESIVES AND WILL VOID ANY GUARENTEE THE ADHESIVE SUPPLIER GIVES U IF YOU DO NOT USE A RECOMMENDED PRIMER WITH THERE ADHESIVE. IF YOU NEED PROPER ADVICE GO TO the tilers forum 

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