First off brush off any loose material from the basement walls.
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Apply small amounts of water over a short period until it has absorbed water deep into the wall.
The walls have been painted and the wall is crumbing in several places behind the paint and salt like particles have formed.
The walls in my 63 yr old full walk out basement are crumbling in spots.
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Now you should apply water to the wall where you will be applying the mortar.
Wash the walls with an acidic solution check on amazon to neutralize the salt completely and allow to dry for a few hours.