What finishing can and cannot do
A raw print usually has visible layer lines, small blemishes where supports were attached and sometimes a slightly rough texture. Finishing, often called post processing, reduces these flaws. It takes patience more than skill, and it adds time. Decide first whether the part needs it at all: a hidden bracket probably does not, while a display piece or a prop might.
Finishing will not fix a badly warped or structurally weak part. It also changes dimensions slightly, so for parts that must fit, finish with care or leave mating surfaces alone, and consider the notes on printed tolerances.
Safety before you start
Sanding plastic makes fine dust that you should not breathe, so wear a suitable dust mask or respirator and work in a ventilated space or outdoors. Wear eye protection, especially when trimming or using power tools. Primers, paints, fillers and solvents often give off strong fumes and can be flammable, so use them in a well-ventilated area away from ignition sources, follow label instructions, and wear gloves. Do not use chemical smoothing methods unless you fully understand the hazards of the chemical involved. A tidy home workshop with good airflow and a place to store flammables makes this safer. Wipe dust up instead of blowing it around.
Step 1: Clean up and trim
Remove supports carefully and clip away stringing or blobs with flush cutters or a hobby knife. Cut away from your body and keep the part on a stable surface. Pare down rough spots with a blade or file before sanding, as this saves effort.
Washing the part with mild soap and water and letting it dry removes oils and dust that interfere with paint.
Step 2: Sanding
Sanding is the main way to flatten layer lines. A general approach:
- Start with a coarser grit to remove the worst ridges.
- Move through progressively finer grits, resisting the urge to skip steps, because skipped steps leave scratches that show later.
- Finish with a fine grit for a smooth surface.
Wet sanding, using water with waterproof sandpaper, controls dust and can improve smoothness on many plastics. Keep the part moving and avoid pressing hard in one spot, since friction can warm the plastic and gum up the paper or deform thin areas. Sanding blocks help keep flat surfaces flat, and folded paper or small files reach curves and corners.
Because the amount of finishing depends on the material, some plastics sand more cleanly than others. Differences between filament types affect how parts behave under sandpaper. Some soften and smear, while others sand into a fine powder.
Step 3: Filling and priming
Even after sanding, gaps and layer lines can remain. Spray filler primer, designed for the purpose, fills fine marks and shows you where flaws are. Apply thin coats, let each dry according to the label, and lightly sand between coats. For larger gaps, a putty or body filler can help, sanded flush after it hardens.
If you find new scratches or pits after priming, repeat the fill, sand and prime cycle. It is normal to do this more than once for a smooth result.
Step 4: Smoothing options
Besides sanding and filling, some makers coat prints with a thin layer of a brush-on or spray coating to fill layer lines. Others apply heat or vapor treatments to certain plastics. These methods can be effective but may deform details, make unpredictable results and involve hazardous chemicals or heat. If you try any of them, test on a scrap piece first, follow the manufacturer’s guidance and prioritize ventilation and fire safety, as discussed in workshop fire safety.
Step 5: Painting
Paint adheres best to a clean, lightly roughened, primed surface.
- Use paint types suited to plastics, and check that the paint and primer are compatible.
- Apply several thin coats instead of one thick one. Thick coats run and hide detail.
- Let each coat dry fully before the next.
- Consider a clear coat for protection, but check it will not react with the paint.
Painting is also a place where color matters to the finished look, an idea touched on in color psychology. Test colors on a spare piece, since paint can look different on plastic than on the can.
Special cases
Parts that touch food or skin. Printed surfaces have tiny crevices where bacteria can hide, and many paints and plastics are not made for food contact. Avoid using printed parts for food unless you have reliable information that the material and finish are suitable, and treat that as a safety question and not a finishing question.
Parts that flex. Hard paint can crack on flexible prints. Use finishes designed to flex.
Outdoor parts. Sunlight and weather can degrade some plastics and paints. A suitable clear coat helps, but results vary.
Reduce the work in the first place
You can lessen finishing effort before the print begins. Orient the part so visible faces are not covered in support marks, and design to avoid support structures where you can. Finer layer heights, as discussed under slicer settings, reduce visible lines, though they take longer. A part designed with a nice flat face on the bed needs little work on that face.
Know when to stop
Finishing can become endless. Decide on the level of quality you need, finish to that level, and move on. A good result usually comes from steady, modest effort at every stage and careful drying times, not from heavy-handed work at the end.