I sealed mine with Tung Oil. Pour it on and rub it in with micro fiber towel. Once it's all absorbed do it again, and again and again until the oil just sits on top. Then it is fully absorbed all the way through the concrete.
Once you get to that point, water will bead on the surface and the concrete will be impervious to anything staining or soaking in. I needed to apply another coat once a year or every year and a half.
I had those countertops for 4 years before I sold the house and never had a stain. Tung Oil seems to fly under the radar when it comes to finishing concrete countertops. I know people who have pulled their hair out using specialty sealing products that don't seem to work for some reason.
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