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Repair any cracks or dips with a fillet seal. Lay drop cloths on the floor and over furniture. Sand the paint and old render off of the wall. Wash the wall down with a wire brush and water. Apply a salt neutralizer to the wall.
Fillet seal is a spreadable sealer that hardens when it dries. You'll want to fill in any cracks or dips on the wall with a fillet seal so you're tanking a flat, smooth surface. Let the fillet seal fully dry before you proceed with the tanking process. Lay down drop cloths adjacent to the wall to make cleanup easier and prevent the tanking mixture from dripping onto the floor. You should also move your furniture outside or to the center of the room and cover them with drop cloths or tarps. When the tanking mixture dries, it will be much harder to remove. Move a hand sander or sandblaster across the surface of the wall to remove the old paint, render, and plaster from the wall. Use a 150 grit sandpaper when working with a hand sander. Continue to sandblast or sand with the hand sander until the wall is smooth to the touch.  This will help the tank slurry better adhere to the wall and will help you in achieving a level coat of tanking mixture on the wall. Wear a face shield and gloves when using a mechanical sander on masonry or brick. Move the brush across the surface of the wall to remove any dust left over from sanding. Then, use a wet rag and wipe down the surface of the wall. You can move onto the next step while the wall is still wet. Purchase a salt neutralizer online or from a hardware store. A salt neutralizer is a clear solution that will neutralize salt in the render and masonry, which can destroy several types of wall finishes if it seeps into the material. Soak a paint brush in the neutralizer and apply it over the entire surface of your wall. As the wall dries, start mixing your tanking slurry.