A part whose vertical walls are clean and whose top face is grainy, ridged, or scored with a repeating pattern of drag marks. No holes and no pillowing — the surface is closed, it is just not smooth.
Why the top is judged harder than everything else
On a vertical wall you see the side of an extruded bead, which is a smooth curve made by the melt's own surface tension. On a top surface you see the upper face of the bead — the side that was in contact with the nozzle as it passed.
That means every irregularity in delivered volume, every place where two beads meet, and every drag from the nozzle plate itself is on show. It is not that top surfaces are printed worse; it is that they are the only surface where the tool touches the finish.
Calibrate flow before anything else
Almost every rough top on a working machine is over-extrusion. The excess has nowhere to go except up, so it piles slightly above the intended height, and the nozzle then ploughs through it on the following pass and smears it.
The tell is that the roughness has a directional grain following the print path, and that the top of the part measures very slightly proud. Print a single-wall test cube, measure the wall, and correct flow — the full procedure is on the over-extrusion page and it is a prerequisite for everything below.
Doing this first also matters because the finishing techniques that follow will happily make a smooth surface out of too much plastic, and then every dimension on the part is wrong in a way that is now hidden.
Then set the top layers up properly
- Slow them down. Top solid layers at around 25 to 40 mm/s look markedly better than the same layers at full speed, and on a typical part they are a small fraction of total print time.
- Use monotonic top infill if your slicer offers it. It forces every top line to be laid in the same direction and in order, so each bead is deposited against a neighbour that is already down rather than into a random gap. It costs nothing and it is the single largest improvement available for top surface appearance.
- Add a top layer or two. More passes over the same area gives the surface more chances to level itself.
- Keep cooling on. A top bead that stays molten while its neighbour lands beside it merges into a wave; one that sets promptly stays where it was put.
Ironing, and when not to use it
Ironing runs the hot nozzle back over the finished top with almost no extrusion, melting the peaks down into the valleys. On a flat top in PLA it produces a genuinely glassy finish.
It has three real costs, and they are why it is not on by default.
- Time. An extra full pass over every top surface, at slow speed.
- It only works on flat, horizontal faces. A curved or sloped top gets nothing.
- It can accumulate material at the edges. The plastic it displaces has to go somewhere, and on some geometries it collects into a raised lip at the perimeter.
Ironing is also actively unhelpful on materials with poor thermal stability, and on any part where the top surface is a mating face — you are deliberately re-melting a surface whose height you care about.
When the fault is somewhere else entirely
If the surface has holes as well as texture, the problem is underneath it and this is not your page — see pillowing. If the roughness repeats at a fixed vertical pitch on the walls as well, look at Z banding. And if the top surface shows a knocked, scarred texture rather than a printed one, the nozzle is striking the print, which is a collision rather than a finish problem and needs attention sooner.
Materials and what they can achieve
PLA irons beautifully and takes the best top finish of any common filament. PETG is harder work: it is stickier at temperature, it drags rather than shears, and ironing it tends to produce a scarred rather than glassy result. Silk grades look superb when they work and show every flaw when they do not. Fibre-filled grades will never be glassy — the filler is at the surface and the finish is inherently matte, which is a property to choose deliberately rather than a defect to chase.