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I am planning to cast an indoor kitchen countertop out of concrete including a concrete sink. Something like this: enter image description here

(the hole is for a small cooktop)

I am planning to use glass fibre mesh for strength, superplasticizer and additives improving water resistance in my concrete mix, however since it is going to have very frequent direct contact with water, I want to seal the surface as well. The problem is that where I live(Klaipėda, Lithuania) I struggle finding any products for this whatsoever. I can easily get PU based clearcoat and silicone based hydro-isolation primer like this one but none of them are designed to specifically seal concrete countertops from water damage so I am hesitant.

I have seen advise to use PVA type 3 wood glue for sealing concrete and my thinking is that since it's water soluble before curing, I can dilute it a lot for first few layers so it gets absorbed by the concrete as much as possible and then use a few increasingly concentrated layers so it bonds really well with the concreate and not only coats the surface. So my thinking is that it should be a pretty good solution and even better than thicker PU based clearcoat that might only form a layer at the surface that could wear off easily. I think I could also use hydro-isolation primer in the same way, but I have not heard of people doing that.

Maybe someone with more experience could advice - is using PVA for this a good idea, or would I be better off using the silicone hydro-isolation primer? Or am I maybe missing something entirely and there are better options I could get my hands on?

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The short answer is no. Countertops near a sink often have water puddles sitting on them for awhile, and a sink is submerged in water. You really need a surface that is at least waterproof, not just water resistant. Even better than waterproof is something rated as submersible. Water resistant PVA won't be suitable long term, and would make it nearly impossible to add a proper coating later.

Another consideration is the characteristics of the surface. A kitchen countertop will get physical wear and tear, so you need a surface that is hard and won't easily scratch or abrade (PVA is soft). It will also be exposed to a variety of water-based and oil-based materials that could stain or discolor it (a problem for PVA), and bacteria and other micro-organisms that can transfer to food. A nearby kitchen window could be a source of UV that could yellow or damage a surface coating over time.

A lot of work will go into making this, and if it isn't done right, it will be an ongoing headache. It may start to look unattractive, and would be a mess to maintain or replace. I wouldn't make this out of concrete if you can't get the right materials to do the project properly. I haven't tackled this personally, so can't recommend the best sealant/surface coating. But there are a lot of online videos covering this, and products specifically designed for this purpose.

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  • Firstly, thanks a lot for response. But how can I trust this answer? What are you basing it on? I find mixed information. Some say PVA type 3 is a good choice for making concrete waterproof, because it soaks deep into concrete, some say it cannot be made waterproof. If you do not have experience in making concrete countertops, how do you know? I guess I'll keep researching and make a few test pieces to test a few options before making the real thing... Anyway, it's not huge, it's cheap and it would not be a terrible loss if this project failed after half a year.
    – Martynas Žiemys
    Commented Jan 5, 2022 at 18:33
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    @MartynasŽiemys, good follow-up questions. I have some experience with concrete and making typical concrete applications "waterproof", but a countertop is different. Concrete, itself, is "waterproof" in the sense that it doesn't redissolve, but it is porous. Water can migrate through it, and internal water can crack the concrete if it freezes. So typical waterproofing is intended to mitigate those problems from "incidental" water, like temporarily getting wet from rain. For an application like a basement wall, where ground water can sit against it for a long time, (cont'd)
    – fixer1234
    Commented Jan 5, 2022 at 19:52
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    especially with the pressure of the water weight, that waterproofing is useless; you need an actual barrier between the water and the concrete. That difference in applications is where the difference in advice comes from. A kitchen countertop doesn't contend with those issues, but you don't want sink water to migrate through, and blended PVA won't help with that. It also needs to deal with the problems outlined in the middle paragraph. If there was no sink, it would be adequate to merely seal the countertop with something that penetrates, (cont'd)
    – fixer1234
    Commented Jan 5, 2022 at 19:53
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    but you would still get staining because those sealants aren't 100%, or don't remain near 100% for long, and a surface coating of PVA is just a poor material for a work surface. There are many concrete countertop sealers available off-the-shelf, but not all are food-safe, and for the sink, you will probably need a resin coating. Here's a video comparing some of the off-the-shelf sealers: How to Seal a Concrete Counter Top. For a sink, you would want a resin-based one. If that coating gets scratched or dinged, you'd want to repair that.
    – fixer1234
    Commented Jan 5, 2022 at 19:53
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    One last thought, PVA is water-resistant but not waterproof. Once it is fully cured, it will resist occasional temporary wetting. But it is still affected by water over time, and it will degrade. It is easily stained, so it doesn't provide great protection against the concrete getting stained. You might get away with it as a sealer for a countertop that doesn't get or stay wet often (as an absorbed sealant rather than a surface coating), but it wouldn't last long coating a sink.
    – fixer1234
    Commented Jan 5, 2022 at 20:26

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