Hold the part at an angle to the light and the infill pattern is legible on the outside: a faint grid, a honeycomb, or a set of diagonal ridges reproducing what is inside the part.
The infill is pushing on the wall from behind
Infill is laid at a different speed and often a different line width from the perimeters, and it is deliberately overlapped into them so the two bond. That overlap means the infill extrusion is pressed against the inside of the perimeter while both are still soft.
A wall is only a fraction of a millimetre of plastic. Push on the back of it before it has set and it moves outward slightly at every point of contact — which is precisely a map of the infill pattern, transferred to the surface where you can see it.
That is why this is not solved by making the walls thicker in the sense people usually mean. It is solved by putting distance and set time between the infill and the surface you are looking at.
The four levers, most effective first
- More perimeters. Three at minimum on any part where appearance matters. The middle wall absorbs the push, so the outer one is not the surface being deformed. This is the change that reliably works.
- Less infill overlap. Stock values are often generous because they were chosen to prevent gaps between infill and walls — the opposite defect. Reducing overlap toward 15 to 20% of line width usually removes the telegraphing without opening a void.
- Slower infill. A fast infill pass hits the inside of the wall harder and carries more heat into it. Bringing infill speed within about half of perimeter speed helps on machines where the two are wildly different.
- Check flow. Over-extruded infill has more material to displace and pushes correspondingly harder. If the part also has a rough top and oversized dimensions, calibrate first and re-evaluate.
The print-order argument
Slicers let you print the outer wall before the inner ones or after them, and the argument about which is correct is genuinely two-sided rather than a matter of one camp being wrong.
Outer wall last is the appearance answer. The outer perimeter is laid onto an already-set inner wall, so nothing pushes on it afterwards and this defect largely disappears.
Outer wall first is the accuracy answer. The outer perimeter is placed onto the previous layer alone, without being crowded by neighbours printed a moment earlier, so dimensions and fine detail come out closer to nominal — and overhanging outer walls have somewhere to anchor.
Choose per part rather than per machine. A display piece wants the first; a mechanical part with tight tolerances wants the second.
A quick way to prove the diagnosis
Print two copies of a small box: one with two perimeters, one with four, everything else identical. If the four-wall version is clean, the cause is confirmed and no further tuning is needed. If both show the pattern equally, the infill is over-extruding and the wall count was never the issue — go and calibrate flow instead of adding walls that cost material on every future print.
Why it is worse on some faces than others
The marking is strongest where infill lines meet the wall at a shallow angle, because a glancing contact runs along the perimeter for several millimetres instead of touching it at a point. That is why a box printed with the usual diagonal infill often shows the pattern clearly on two faces and barely at all on the other two.
Rotating the infill angle in the slicer moves the problem rather than removing it, which is occasionally exactly what you want — on a part with one visible face and three hidden ones, turning the infill to meet the visible wall squarely is free and takes one setting.
Two overcorrections worth avoiding
Do not drop infill to nothing. A part with no infill has an unsupported top surface, and you will trade a cosmetic flaw for pillowing, which is much worse.
Do not raise wall count to six and leave it there. Perimeters are the most expensive material on the part per unit of visible surface, and beyond three or four you are paying for print time rather than appearance.
Where the material comes in
Any translucent or lightly pigmented filament shows this far more, because the pattern is visible through the wall as well as on it — natural, clear and light-coloured PETG are the classic examples, and a translucent part will telegraph its infill no matter how many perimeters you use. Glossy surfaces show the deformation optically; matte and fibre-filled grades hide it. If a part must be translucent and clean, the answer is to print it solid rather than to tune it.