Walls have smoke damage from homeless setting fire inside. What do I n

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Clean the walls with tsp, let dry. Then prime with a stain blocking primer, let it dry throughly, then paint. If you don’t stain block the paint job you could be seeing shadows of the smoke damage

https://www.accutechrestoration.com/cleaning-smoke-and-fire-damage-from-walls-ceilings/

If the fire was inside the walls on studding. Replace the stud's. If your you don't everytime it gets humid you will smell the smoke smell.

You will have to clean, then use a product called Kilz which will coat the smoke damaged area and seal in the odor and prevent the soot from bleeding through. Then repaint.

Is this property insured? Contact your insurer.