If a freshly washed bonnet still feels rough under your fingertips, that is bonded metal in your clear coat — not dirt. It needs dissolving out, not scrubbing.
Industrial fallout is fine airborne metal — from rail lines, brake dust, factories and cutting work — that lands on horizontal panels, then corrodes into the clear coat. It shows up as tiny orange or black specks on light paint, and as a gritty texture on any colour.
Fallout particles are physically embedded. A wash mitt slides over them, and scrubbing harder just drags the particle across your paint and scratches it. The only correct sequence is to dissolve the iron chemically, then remove what is left with a clay treatment, then assess whether the paint underneath needs correcting.
Cars parked near rail corridors, industrial estates or busy freeways collect this constantly. Once the paint is coated, fallout has a much harder time keying in, and what does land tends to come off in a normal wash.