You brush the basement wall clean, move the shelving back, and find the same white powder after the next wet week. Before buying paint, photograph the patch. The useful question is where the moisture came from, not how quickly you can hide the mark.
Treat the deposit as evidence
Natural Resources Canada lists efflorescence among signs of foundation dampness and advises resolving moisture before interior insulation. Photograph its location and recurrence. A surface that appears dry on inspection day may still have a moisture history worth investigating.
Do not leap from a mark to a whole-house solution
Check the wall's relationship to exterior grading, downspouts, cracks, and known leaks with an appropriate contractor. Ask which observation supports the proposed repair. A mineral deposit alone does not tell you whether an interior measure or excavation is needed, nor does it identify every white material on a wall.
Before the next step
- Record the mark before cleaning.
- Compare it after rain and seasonal changes.
- Resolve the moisture cause before enclosing the wall.
Leave the affected patch visible while the cause and repair are being checked. Shelving can wait; opening a finished wall again is a less convenient way to inspect it. Include the sump discharge route in the drainage review.