You can strengthen, insulate, waterproof and beautify porous walls built of cinder blocks by adding a thin coating of surface bonding or resurfacing cement. The critical issue is creating a strong chemical bond between the cement in the cinder blocks and the new cement surface. The cement itself is the glue that creates the bond. When activated by water it will "grow" the tiny crystals that create the connection. For best results, resurface your wall during moist – but not wet – weather, and keep the new cement surface damp while it cures.
Things You'll Need
Trisodium phosphate (TSP)
Surface bonding/resurfacing cement
Batch mixer, or a drill and paddle
Spray bottle of water
Stainless-steel finishing trowel
Clean the cinder block wall, scrubbing it with trisodium phosphate (TSP) solution, then thoroughly rinsing it. Brush off any loose "cinders" and vacuum if necessary. You want the wall to be clean, but not dry.
Mix the surface bonding cement or wall resurfacing cement with water – adding the cement to the water – with a batch mixer, following manufacturer's instructions. Alternatively, mix it in a large tub using a drill and mixing paddle.
Thoroughly moisten the wall with water from the spray bottle. Fill the plaster tray with cement mixture. Load the finishing trowel with cement and evenly coat the cinder block wall, starting at the bottom of the wall and working up.
Apply a layer of resurfacing cement 1/8-inch thick, using an upward motion. Push the cement into any cracks or broken areas. Texture the surface as you go.
Cure the new wall surfaces by keeping them moist for at several days. Mist the walls with water from the spray bottle every eight hours, or three times every day, for as many days as the manufacturer recommends.
Do not resurface cinder block walls if temperatures will be hotter than 95 to 100 degrees Fahrenheit or colder than 40 degrees within the next 24 hours. Cool and moist -- but not wet – weather conditions are ideal.
Don’t let the new surface dry out too soon, which may cause the cement to crack or the bond with the cinder block to fail.