We will need to have a reasonably smooth surface, because we will later be applying a paste-type filler/sealer with a trowel or squeegee.  This means there should not be little bumps of concrete sticking-up along the edges of the mortar-joints between the concrete blocks.

Run your hand over the surface.  If it feels smooth, you are ready for the next step.  If you feel bumps along the mortar-joints, those should be ground flat.  A 36-grit aluminum-oxide sanding-disc is a good way to knock down those high points.  Be sure to use an aluminum oxide or silicon-carbide type of abrasive.  Garnet, commonly used for sanding wood, is too soft an abrasive to grind down sand- and gravel-cement bumps.

Next, the concrete surface needs to be cleaned.  Dust,  dirt, cobwebs, powdery residues (efflorescence) dried salt residues as well as the sanding-debris from the earlier step, all these can usually be removed by scrubbing with a bristle or wire brush and then vacuuming the surface. 

Large holes should look clear to a depth roughly equal to the diameter for good results.  Remove loose dirt and dust in the holes in the concrete blocks, as that can make the rest of the process less effective.

The cement surface must be reasonably dry before continuing.

Moving air is the most efficient means of evaporating water.  You must cause fresh dry air to come in from the outside, blow it against the walls and carry the humid air out of the basement.  Placing exhaust fans in outside windows and arranging circulation-fans to blow inside air against the walls is an effective arrangement.

Now, we are ready to begin product application.  There are two products needed to perform the sealing of the concrete blocks, a liquid and a paste. The liquid is the Damp Concrete Primer (DCP).  It is prepared by mixing a liquid-concentrate with water.  A tan emulsion will immediately form.  You must use this within about 30-45 minutes.  The microscopic droplets of the resin-in-suspension will soak into the large, medium and small concrete pores, but will not get into the really microscopic porosity that concrete usually develops.  In the case of concrete blocks there really isn’t any; the porosity of this material is for all practical purposes the very large pores between the gravel-grains that are stuck-together by the sand-cinder-cement mixture (which is why they are also known as cinder-blocks).

Start in the morning of a day.

Apply the Damp Concrete Primer with a roller, holding the roller horizontally and rolling upwards.  The liquid will wet the surface and flow into the open porosity.  It is not necessary to fill all the holes with the liquid or get it really deep; if the surface and the insides of the holes you can see are wet, that’s good enough.

The water-component of the DCP will evaporate over the next hours, and the surface will have a clear brownish oily film.  Over the next hours it will go from slippery-wet to sticky-wet to dry-to-the-touch but still soft-to-the-fingernail. When it is no-longer-sticky, that’s the time to apply the Fill-It Epoxy Filler; in the afternoon of the same day.

The next step is to smear the filler against the surface so as to force it into all the holes in the concrete block.  Long ago, cement mortars were used as thin cement coatings that cure quickly (and appear to solve the problem by covering it up), but most do not develop good adhesion to the cinder-block surface, allowing continuing water-seepage; later they usually crack and break off.  This is why the modern epoxy and urethane adhesives and fillers are used.  Smith’s Fill-It Epoxy Filler has been used for decades to do this kind of repair.

Squeegee the filler over the surface right-to-left and then left-to-right; then upwards and then down.  A concrete block done in this manner will have the holes permanently sealed.

Below-grade repairs can be problematic, in the wet season.  Slightly damp concrete can be tolerated, but water leaking in from a high-water-table outside cannot.  The reason is that the outside water-pressure will tend to lift the filler off the surface before it and its primer can cure.

The concrete blocks can be squeegeed or scraped completely smooth.  It is not necessary to leave a glaze or troweled coating on the surface.  The typical coverage of a filler used in this manner is thirty to a hundred square feet per gallon, depending on the porosity of the concrete blocks.  A typical 8" X 16" concrete block is about nine-tenths of a square foot.  You may find most of the filler ends up in the dished-out mortar joints between the concrete blocks.  If you want the original mortar-joint appearance, you can go over the finished area with a wooden tongue-depressor (or other similar tool) before it cures and clean out the filler and reapply elsewhere.  The working-time for this kind of job is typically an hour, so mix only enough filler as you can efficiently use in an hour.

The filler goes on in the afternoon of a day.  Areas primed with the Damp Concrete Primer but not-yet-filled will need to be primed again the morning of the next work-day.

Please do not get any epoxy products on your bare skin, and do not to breathe the smell of any epoxy or its hardener.  Have a fan at your back if in a confined area.  Use these or any chemical products with adequate ventilation, and use gloves when working.  Even if the labels on the containers don’t say, all amines and all epoxy resins are chemical irritants at best, meaning are hazardous if you get them on your skin a lot (or sometimes for some people, only a little).  They are quite safe when used wisely, in accordance with proper safety procedures.

----Steve Smith

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