Stucco over painted block,

Ask the pros-August, 2006
By Reggie Bullard

  Stucco over painted block, brick or stucco by chipping the block

Chipping up the wall is a variation of an old method that seems to work the best for us. Sand blasting is a real chore and not cheap. Usually, sandblasting leaves a dust cloud that looks like a fire in the distance. 
chipping paint off
                                    block chipping paint off
Here paint is chipped off using a chipping hammer. A good place to buy these is from Harbor Freight. Deep holes don't matter. They have a better key for the mortar. A race between a hatchet and an electric chipping hammer shows the hatchet is almost as fast. It is good to have a grinder on the job to sharpen the hatchet or bits once in a while.
paint chipped off
                                    block flex-con,
                                    acrylic bonding admixture for
Ready for mortar. Chips leave areas both rough and porous for a permanent  bond. Flex con is used for additional adhesion. 
We mixed flex con half and half with water and mixed it into the dry mixed mortar. 
I have to advertise also for Milestone.  Milestone "E" chemical is concentrated  and will make the equivalent of 3 buckets of flex con, diluted with water. 
The acrylic not only is adhesive for a good bond, but is resistant to cracks and  adds strength.
stucco dry mix and wet mix applying bonding coat
                                    of stucco mortar
Here the dry mix is mixed with the wet mix with a drill, but a hoe works, too. Scratch coat is applied with a lot of pressure to give a good squish into the holes. Scratch coat is scored and allowed to set 2 days before the brown coat. How long will this last? I think about 
4,000 years.
Paint on bonding agents seem to fail over time, At least all the work we have done with them on the outside (over painted surfaces) eventually popped off the wall. The reason paint on agents fail on the outside of a building is because they dissolve in water. The reason they work to begin with is that the surface dissolves when wet plaster or cement mortar is applied. The mortar then mixes with the bonder and dries together, forming a chemical bond. 
         We have had a lot of cracks nailing metal lath over the painted block. I think the reason we have excessive cracking with lath is the paint forms a “slip joint”. The stucco expands and contracts one way and the block or brick another way, causing a lot of cracks. Nailing lath on block is also a slow and expensive process. Even with stub nails or pin set pins the lath doesn't seem to be attached as well as putting the mortar right on the block. Nailing brick is unreliable. The nails break 
up the brick,  leaving the only resort of driving nails into the mortar joints. 
          Most of the area of the block is chipped up, leaving the pores open on the blocks for a good bond.   A few holes don't hurt, mortar squished into these holes form a good key for the slab. We have had very few or no cracks at all using this method. 

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