Ash is not dust
Wildfire ash consists of angular particles of burnt mineral and carbon. Under magnification it looks like broken rock, not like the rounded particles of road dust.
Dragged across glass under pressure, it cuts. That is the whole problem.
What not to do
Do not run the wipers. Dry-wiping ash is dragging abrasive across the swept area under spring pressure, and it produces exactly the arc-shaped scratches that cannot be removed.
Do not brush it off with a dry cloth or a duster, and do not use a dry car cover — it grinds the ash into the paint and the glass as it moves.
What to do instead
Rinse with plenty of low-pressure water first and let it carry the ash away before anything touches the surface. Only then wash normally.
Inside, the cabin filter loads up fast during ashfall and is worth changing after a heavy week. And if the car has tint, be especially careful on the inside of the rear window — the film and the defroster grid are both on that surface, and both are easy to damage with an abrasive cloth.
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