Concrete Basics

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Staining Concrete

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KILZ "Kills" Stain Penetration

Question:

We converted our garage into two bedrooms. I just put the first coat of stain on the concrete floor. The day before we stained it, I pressure washed the concrete with a little Joy dishwashing liquid. When we cleaned the floor, we found a few spots of paint on it, a brand called KILZ. We got the paint up, but those spots did not take the stain; they are still gray concrete. Why didn't those spots take the stain?

Answer:

KILZ, as well as all other primers and paints, are comprised of different chemicals. Some of these chemicals are designed to promote adhesion by bonding deep into the surface being coated. The surfactants, resins and oils -- if it's an oil-based system -- will actually soak deep into the surface. The porosity of the surface (concrete is considered a very porous material) will dictate the depth of penetration and the strength of adhesion. In your case, the resins and oils soaked into the concrete, which is why you still have a stain mark, even after the surface coating was removed. Resins and other chemicals in the KILZ leached out of the coating and into the concrete. They are typically not visible when the surface is dry, but show up when wet, as they resist the water from penetrating. To remove the spots, you need to use a soap designed to cut heavy oil or grease to emulsify the contamination, and then you need to soak it up. Depending on the length of time the stain has been on the concrete, this can be a quick process or a long, slow process.

A paint drip, even after being cleaned, retards stain
from penetrating into the concrete.

As a side note, since your floor has already been stained, and there are only a few spots, it's often best to just leave them alone as "character marks" in your stained floor. Attempting to remove them now that the floor has been stained can actually create a bigger spot that ends up looking worse than the original.

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