A surface defect is evidence. Before changing anything, look at the geometry of the marking itself, because where a defect appears on a part tells you far more than what it looks like up close.
Four questions sort almost everything in this category.
Does it repeat at a fixed spacing?
Regular horizontal bands, evenly pitched up the whole part, mean something is repeating on a cycle. A leadscrew turns once per fixed vertical distance; a bed heater cycles on a thermal period; a driver produces the same tiny motion error every full motor step. Measure the spacing with calipers and compare it against the leadscrew pitch. That single measurement separates a mechanical cause from a flow cause, and no slicer setting will touch the mechanical one.
At the other end of the size scale, a shimmering scale-like texture that has no measurable pitch at all is a driver artefact rather than a motion fault, and it is partly inherent to the electronics rather than a defect you introduced.
Does it start at a corner and fade out?
Ripples that trail a sharp direction change are inertia. The toolhead has mass, the frame has stiffness, and the two together have a natural frequency the machine excites every time it stops abruptly. The defect fades as the oscillation damps, which is why the wall is clean again a couple of centimetres later. Acceleration, not speed, is what excites it — a fact that surprises people who slow the whole print down and see no improvement.
Does it appear where the nozzle stopped, started, or merely passed over?
Melt pressure does not vanish when the extruder stops turning. Anything that shows up at a start point, an end point or along a travel path — a raised bead, a vertical scar up one face, hairs between towers, a deposit on a surface the nozzle only crossed — belongs to that family. They are all the same physics viewed at different moments, and they respond to the same small set of levers: retraction, pressure compensation in the firmware, temperature, and where the slicer chooses to put the join.
Did the plastic have anything underneath it?
Overhangs, bridges, top skins over infill and the top of a support tower all ask molten polymer to hold a shape with nothing supporting it. Whether it succeeds is a race between solidification and gravity, so cooling airflow and how fast the nozzle crosses the unsupported span matter more than anything else. This family is also the one where reorienting the model beats every setting available.
The ones that are really flow faults wearing a surface disguise
A grainy top face, an infill pattern printing through onto an outer wall, a wall whose thickness visibly wanders along a straight run — these look like finish problems and are usually volume problems. Calibrate flow before spending an evening on ironing settings, or the finish work is compensating for something that should have been corrected upstream.
What is honestly not fixable
Two things in this category have a floor. A face that was printed against supports cannot match a face printed against air, because it was built onto a deliberately weak surface. And on some driver and stepper combinations a faint fish-scale sheen persists no matter what is changed. Both are worth knowing before an evening disappears into them, because in each case the productive response is a decision about the model rather than a decision about the printer.
How to work through it
Photograph the defect in raking light, change one variable, print the same test object, and compare against the photograph. Surface problems tempt people into changing five settings at once more than any other category here, because every one of them has a plausible-sounding candidate fix.