Post-processing 3D prints: sanding, filling, painting

Highlighted terms are explained briefly when you hover over or tap them.

A print almost never comes off the bed looking the way it should in the end. Layer lines, support remnants and small gaps stand out especially when a part is painted or put on display. With sandpaper, filler and paint you can turn it into a surface that hardly looks 3D printed any more. Here is the order that has proven itself.

Beforehand: less finishing through good settings

Every layer line you do not print is one you do not have to sand away. A smaller layer height and a good orientation on the bed save a lot of work. For flat top surfaces the slicer also offers ironing: the nozzle passes once more over the top layer with a little material and smooths it. More on this under Holes in the top surface.

Step 1: remove supports

Remove supports with pliers, a cutter or a deburring tool and take off the remnants roughly. How to do this cleanly, and how to set up supports so that they come off easily, is covered under Supports.

Step 2: sanding

The starting grit depends on the layer height:

Then work through finer grits step by step, for example 220, 400, 600. Do not skip a step, otherwise the scratches of the coarser grit stay visible.

Wet sanding is almost always better with plastic. The water cools and binds the dust. Dry, PLA quickly softens from the frictional heat and smears, PETG even more so. Use waterproof sandpaper, add a drop of washing-up liquid to the water, sand in circular movements with light pressure and rinse off the sanding slurry from time to time.

For flat surfaces use a sanding block, otherwise you round off edges. For curves and details, sanding sponges and files help.

Sand fibre-reinforced filaments with carbon or glass fibre only wet and with respiratory protection. Why is explained under 3D printing safety.

Step 3: filler and putty

Deep gaps, seams and glue joints are closed with filler. For small spots, fine filler from model-making or car supplies is enough. For larger areas, 2K filler is suitable, which hardens chemically and does not shrink afterwards. Apply thinly, let it cure and sand flush with 220 to 400.

Step 4: filler primer

A filler primer is a thick primer that fills fine layer lines. How to work with it:

1. Degrease the part, for example with isopropyl alcohol.
2. Spray two thin coats from a distance of about 15 to 20 cm. Several thin coats are better than one thick one, otherwise drips run and details disappear.
3. Let it dry, with the time given on the can.
4. Wet sand with 400. The primer stays in the recesses while the peaks are removed.
5. Repeat until no layer lines are visible. Before painting, wet sand with about 600.

Grey primer shows flaws best.

Step 5: painting

Spray paint is the easy way. Acrylic paint from a can adheres well to a primer. Here too, several thin coats. Do not spray in high humidity or cold, and follow the manufacturer’s information on processing temperature.

An airbrush is worthwhile for figures, colour gradients and fine details. It needs a compressor, a gun and practice, but you save paint and have more control.

Important with both: always apply solvent-based paints outdoors or with good ventilation, and wear respiratory protection with a suitable filter.

Step 6: clear coat

Clear coat protects the paint from abrasion and UV. It comes in matt, satin and gloss. Two thin coats are usually enough. Make sure the paint and clear coat match, ideally from the same system. If in doubt, test on a sample piece whether the clear coat dissolves the paint.

Gluing

Before gluing, lightly sand the surfaces and degrease them with isopropyl alcohol.

• Superglue (cyanoacrylate): fast, good for small parts made of PLA, ABS and ASA. With PETG it often holds less reliably. Gel superglue fills small gaps. An activator spray speeds up curing.
• 2K epoxy: for larger surfaces and loaded joints. Fills gaps, but depending on the product takes minutes to hours. Suitable for almost all rigid filaments.
• Solvents: only for ABS and ASA. Acetone dissolves the surfaces and the parts fuse almost invisibly. With PLA, PETG and nylon this does not work. Acetone and ABS scraps can also be mixed into a paste for gluing and filling.

Acetone is highly flammable and the vapours are harmful to health. Work only with good ventilation, no ignition sources nearby and with gloves.

Which glue holds on PETG and TPU, and how to prepare the surfaces, is covered in detail under Gluing 3D prints.

Smoothing

Acetone vapour for ABS and ASA

Acetone vapour dissolves the surface of ABS, the layer lines blend together and the part turns glossy. It also works with ASA, often less evenly. Details and sharp edges become softer, and holes and fits can change. Exactly how this works and which safety rules apply is explained under Printing ABS. Never heat acetone. The vapours are flammable and can form an explosive mixture with air.

Epoxy coating

Thin epoxy resin applied with a brush fills layer lines and gives a hard, glossy surface. This works regardless of the filament, including PLA and PETG. Mix thoroughly and without air bubbles and work quickly, as many resins become tacky after about 15 minutes. Running resin collects at lower edges, so turn or hang the part while it cures.

Heat

A heat gun on a low setting removes fine strings from stringing in seconds. For smoothing whole surfaces, however, heat is unsuitable: the part warps before the surface becomes smooth.

What works with which material?

The values are guide values. Filaments differ depending on the manufacturer and additives, so test new combinations on an offcut.

Continue with: Melting in threaded inserts if your parts are to be screwed together.