The part comes out of the wash bay with fine lines that were not there when it went in. They are usually on the most visible face, they follow the length of the part rather than crossing it, and they show up worst in low, raking light when the panel is dry.
Three different things are being confused in that description. Material may have been removed, something may have been deposited, or the surface may have been marked all along and the wash only took off the film that was hiding it. Each has its own signature, and the signature is what you read.
A brush, sponge or mitt that has been used on wheels and lower panels holds grit, and grit dragged across a finished surface cuts a line. The result is a family of roughly parallel scratches, all in the direction of travel, gathered wherever the hand or the brush changed direction and pressed harder.
This is why the damage concentrates on flat, easy-to-reach faces and stops dead at an edge, a bracket or a weld. The wash did not attack the metal. It dragged something across it, and the something came from the car.
Automatic equipment writes a different signature. Brushes rotate, so the marks curve, and they sit at one height across the whole width of the car. Guide rails, conveyor lugs and low pressure nozzles leave straight lines at bumper height. Nothing about those marks follows the shape of your component.
So compare the marks with the geometry. If a scratch runs across a weld, over a bracket and onto the painted bodywork in one continuous line, the cause was outside the car. If every mark stops at the boundary of the part, look instead at how the part was handled before it went in.
A dusty metal part looks uniform. A wash removes the film, and every existing mark appears at once, which reads as damage done during the wash. Handling scratches from fitting, a wipe with a gritty cloth during a service, a bright ring left by a clamp being slid into position all live under that film for months.
Then separate a scratch from a stain. A chemical mark from an acidic wheel cleaner or a strong alkaline pre-wash dulls a patch without cutting it. It looks like a scuff while the surface is wet and then partly disappears as it dries, which no mechanical scratch ever does.
Drag a fingernail across a mark at right angles to it. If the nail drops into a groove, metal has been displaced and the mark is mechanical. If the nail runs over it smoothly, the surface is intact and you are looking at a deposit, a stain, or a change in the oxide.
Then use the light. Wet the area and look from a low angle, and repeat once it has dried. A mark that vanishes when wet is sitting proud of the surface. A mark that is stronger when dry and fainter when wet has depth beneath the surface, and will need work.
A shallow scratch in a brushed finish can be re-brushed along the existing grain, and a polished face can be brought back if enough material remains. Deep cuts that leave a bright line under the light cannot be hidden; they have to be blended into the surrounding finish by someone who works the whole area, not just the damage.
Prevention is cheaper than either. Wash finished metal last, with a fresh mitt, and never with the media that touched the wheels. On a titanium fabrication component the finish is the surface itself, with no coating to renew, so a light hand matters more than a strong product. Finish also belongs with material choice rather than after it, which is how our shop by material pages are laid out.
