Sanding and smoothing

Run a fingernail sideways across a printed wall and you feel them: fine ridges, one per layer, marching up the part like a contour map. Those layer lines are the signature of FDM, and they're also the thing standing between your print and looking like it wasn't printed at all. Sanding is how you take them off. It's slow, dusty, satisfying work — and if you understand what you're actually doing, it's also work you can plan for in the model so you never sand away something that mattered.

Layer lines are hills and valleys

Each layer is a rounded bead of plastic sitting on the one below, so a printed surface isn't flat — it's a row of tiny parallel hills with valleys between them. Sanding knocks the tops off the hills. But if you only knock the tops down, you're left with the valleys, and the surface still catches the light unevenly. The real goal is to bring the whole surface down to the depth of the valleys, or to fill the valleys up to the hills. Usually you do a bit of both.

That's why you sand up through the grits. A coarse paper (say 120–180) cuts fast and removes the hilltops quickly, but it leaves its own deep scratches. A medium grit (240–400) removes those scratches, leaving finer ones. A fine grit (600–1000+) removes those. Each step only has to erase the scratch pattern of the step before, which is why skipping grits doesn't work — a fine paper can't remove a coarse scratch in any reasonable time, it just polishes the ridges. Wet-sanding, with a little water on the paper for the finer grits, floats the dust away so the paper keeps cutting instead of clogging, and it keeps the plastic from heating and gumming up.

Filler primer does the other half

Sanding removes material to reach the valleys; filler primer goes the other way and fills the valleys up. Sprayed on, it's a thick, sandable paint that settles into the low spots. Sand it back and the high spots show through bare while the primer stays in the dips — you've levelled the surface without removing much plastic at all. A cycle or two of prime, sand, prime, sand will bury layer lines that would take forever to sand out directly, and it's the standard route to a genuinely smooth face. It also sets you up for paint, which is where this leads.

The part you sand is smaller afterwards

Here's the fact that ties this to your model: sanding removes material, so a sanded face ends up smaller than you drew it. Take half a millimetre off a wall and that wall is half a millimetre thinner; sand down a peg and it's a hair narrower. That's fine on a cosmetic outer surface and a disaster on a face that has to fit something.

So budget for it. When you know a face will be sanded and smoothed, draw it with a sacrificial skin — 0.2 to 0.5 mm of extra material you fully intend to remove. You sand into the skin, not into the real dimension, and the part lands on-size when you're done. The flip side of the same rule: never put a critical dimension on a face you'll sand. A bore that has to hold a bearing, a slot sized for a fit, the mating faces of two halves — leave those as-printed and smooth everything around them. If a fit surface and a show surface are the same face, split them in the model so you can finish one and protect the other.

Sanding gets you matte-smooth, but it can't give you a glassy, sealed shine — for that you melt the surface instead of abrading it. That's Vapour and chemical smoothing.