Block walls weep because they are built to. CMU is affordable precisely because it uses less cement, which leaves wide interstitial gaps and hollow cores that drink groundwater and hand it to your basement. A sealer or waterproof paint on its own will not fix it — Bone Dry sealers work by reacting with the alkali in a substrate, and pervious block does not carry enough to feed the reaction. The Bone Dry Block Wall system solves it in two steps from the inside of the wall: Bone Dry Primer penetrates deep into the block, fills the voids, and deposits the alkali the sealer needs; then Bone Dry Original or Bone Dry Plus goes on while the primer is still damp and reacts with those deposits to form a permanent moisture barrier inside the block itself. No excavation, no membrane, nothing on the surface to peel.
How do you waterproof a concrete block basement wall from the inside?
Why block walls weep — and why sealer alone cannot stop it
Soil and hydrostatic water press against the hollow cores of a below-grade CMU wall. Poured concrete has fine capillaries and plenty of free alkali, so a Bone Dry sealer reacts fully and works as designed. A block wall has wide interstitial gaps and runs short on alkali, so the same sealer cannot fully react on its own. The Primer tops up the alkali; the sealer then fills every small space within the block. Applied together, the barrier forms inside the wall before water ever reaches the room.
The two-step system
| Step | What it does |
|---|---|
| 1 — Bone Dry Primer | A colorless, non-toxic, water-based blend of inorganic suspended solids. Sprayed on the interior face, it penetrates deep into the block, filling hairline cracks, small voids, and the interstitial spaces between matrix particles — and depositing the alkali the sealer needs. Two coats at 80–150 sq ft per gallon, depending on porosity. |
| 2 — Original or Plus sealer | Applied while the primer is still damp. Reacts with the primer deposits to form a permanent moisture seal in the block. Choose Plus for antimicrobial protection in a mold-prone or musty basement. |
Which walls need the primer
| Your surface | Use |
|---|---|
| CMU block wall, shotcrete, gunite, or extremely porous concrete | Bone Dry Primer, then Original or Plus — always prime first |
| Standard poured concrete wall or slab | Original, Plus, or Pro directly — no primer needed |
| Painted block (latex, acrylic, or polyvinyl paint) | Strip the paint first — the Primer cannot penetrate it |
Sealing the floor as well? The slab takes Original or Plus directly — see Sealing a Below-Grade Concrete Slab. For the whole line, see the Bone Dry product selector.
Before you start: what the Primer will not do
- Painted walls. Bone Dry Primer will not penetrate latex, polyvinyl, or acrylic paints, or nonporous rubber-based paints. Strip them so the Primer can reach the block.
- Coated or contaminated surfaces. Heavy wax, thick grease, a recently applied surface sealer, or other impervious material must come off first. Oil, grease, or acid conditions need cleaning with a commercial degreaser — scrape heavy deposits — before the Primer goes on.
- Not a concrete material. The Primer is formulated solely for cement-based building materials. It does not effectively penetrate asphalt, metal, wood, or clay brick, and it should not be applied to masonry with a mortar or grout containing a latex binder.
- Glazed surfaces. Do not apply to glazed tile or glazed, hard-fired brick — the glaze blocks penetration. The exception is sealing grout joints, where excess is removed from the glazed faces with a wet vac, squeegee, or mop once the Primer has soaked into the grout.
- Cold walls. Minimum surface temperature is 45°F. Do not apply or store frozen; if it freezes in storage, agitate the thawed material thoroughly before use.
- Diluted product. The Primer must be applied at full strength to attain the desired results.
How to apply the Block Wall system
- Prep. The wall must be unpainted and clean. Strip paint; degrease oil or grease; remove any wax, thick grease, or recent sealer.
- Mist. Dampen the block with a fine water mist — damp, not saturated.
- Prime, twice. Saturate the wall with Bone Dry Primer at full strength, two full coats at roughly 80–150 sq ft per gallon depending on porosity. A hand-pumped garden sprayer works well for medium areas, airless spray for large ones, a spray bottle for small spots. Brushes and rollers are not recommended.
- Let it reach damp. Allow the substrate to dry to a damp condition. If white deposits form from over-application, brush them off or flush with water before the next step.
- Seal while damp. Apply Original or Plus once the Primer reaches damp, and the sealer reacts with the deposits to form a permanent barrier deep in the block.
Working outside on a foundation wall, protect vegetation and adjacent areas from direct spray or overspray. As with all Bone Dry sealers, wear goggles or a face shield and gloves for the sealer step, and use a NIOSH-approved respirator in confined areas — a basement counts. The Primer itself is non-toxic, non-flammable, has negligible odor, and will not discolor the surface.
Because block is pervious by design. CMU uses less cement than poured concrete, which leaves wide interstitial gaps that drink groundwater, and it does not carry enough free alkali for a penetrating sealer to react with. Paints and film-forming waterproofers sit on the surface and eventually lift as water pushes from behind. The Bone Dry Block Wall system supplies the missing alkali with a Primer, then seals inside the block with Original or Plus.
No. The Bone Dry Block Wall system is applied from the inside face of the wall with a garden sprayer. The Primer penetrates deep into the block and the sealer forms the barrier inside it, before water reaches the room — no excavation, no exterior membrane.
No. Bone Dry Primer will not penetrate latex, polyvinyl, acrylic, or nonporous rubber-based paints. Strip the paint first so the Primer can reach the block.
That is over-application: the Primer left excess solids on the surface. Remove them by brushing or flushing with water before the next coat or the sealer goes on. Accidental over-application will not discolor the wall.
Both react with the Primer deposits to form the same permanent seal. Choose Plus in a mold-prone or musty basement — it carries antimicrobial technology that prevents mold, mildew, and odor-causing bacteria in the pores. Original is right for a standard wall without a growth problem.
Usually yes — the floor is the other path for ground moisture. A poured concrete slab takes Original or Plus directly with no primer. Send us the wall and floor square footage together and we will size the Primer and sealer as one system.
Yes. Shotcrete and gunite are extremely porous like CMU, so they are always primed first, then sealed with Original or Plus. Standard poured concrete does not need the Primer.
