A print straight off the build plate is a starting point. This guide is for anyone curious why some pieces look molded and others show their layers. It covers the settings, sanding, smoothing, priming and painting that take a print from rough to smooth, and where we draw the line on our own pieces.
Layer lines are a feature on some pieces and a flaw on others
Layer lines are the signature of FDM printing. Each horizontal line is where one layer ends and the next begins.
Layer lines help on:
- Articulated toys, where the texture feels good in the hand
- Designs that deliberately show how they were made
- Pieces where a handmade look is the point
- Textured designs that hide or use the lines
A smooth finish is better for:
- Display collectibles meant to look factory made
- Pieces imitating wood, stone or metal
- Pieces that will be painted realistically
- Products where polish signals quality
We decide per design. Many articulated dragons look better with their layers showing, while display pieces often get smoothing and finishing.
Smooth finishes start with the print settings
The better the raw print, the less work afterward.
Layer height
Fine layers, 0.1 mm to 0.12 mm, show less stepping between layers. The cost is print time, often double or triple that of standard layers.
Standard layers, 0.16 mm to 0.2 mm, balance quality and speed, and are our default when a piece does not need to be exceptionally smooth.
Thick layers, 0.24 mm to 0.32 mm, print fastest with pronounced lines, and suit functional parts where looks are secondary.
For decorative pieces that will be smoothed, we usually start at 0.12 mm or 0.16 mm, which cuts down the heavy sanding later.
Speed and temperature
Slow outer perimeters, 30 to 40 mm/s, give cleaner surfaces because the filament is placed more precisely and bonds better.
The right temperature avoids blobs from over-extrusion and gaps from under-extrusion. Each filament has an ideal range, and hitting it gives the cleanest raw surface.
Cooling and orientation
Enough cooling prevents the sagging and oozing that mar a surface, especially on overhangs.
Orientation can put layer lines on the faces nobody sees while keeping strength and detail where the piece is viewed.
Sanding is the most common way to smooth a print
Working up through finer grits produces a surface ready for primer or for use as it is.
The grit progression
Coarse, 80 to 120 grit, removes major layer lines and support marks but leaves visible scratches. Use it sparingly so you do not take off too much material.
Medium, 150 to 220 grit, removes the coarse scratches and keeps reducing the layer lines. Most of the work happens here.
Fine, 320 to 400 grit, leaves a surface that feels smooth, with layer lines faintly visible at most.
Very fine, 600 to 1000 grit, gives a near-glass finish, used before painting or as the final finish on bare plastic.
Wet sanding is essential above 400 grit. Water keeps friction heat from melting PLA, carries away dust and gives a finer finish.
Techniques
Hand sanding with paper wrapped around a foam block or sponge gives control on curves and detail, because foam follows the shape better than a rigid block.
Power sanding with a rotary tool or orbital sander speeds up large flat areas but can dig dips or remove too much.
Contour sanding with curved blocks, dowels or a sandpaper-wrapped finger keeps the shape right on organic forms like dragons and animals.
Finish each grit before moving on. Switching to a finer grit too early leaves scratches that get harder to remove.
Getting a complex piece glass-smooth can take hours of careful work, which is why finished pieces cost more than raw prints.
Chemical smoothing melts the surface flat
Solvents partly dissolve the plastic surface so it reflows smooth. It can do what sanding cannot, and it needs proper ventilation and safety equipment.
ABS and acetone
Vapor smoothing puts an ABS print in a chamber of acetone vapor. The surface softens and reflows, and the layer lines disappear.
Brush-on acetone smooths specific areas with more control than a vapor bath, but needs a steady hand.
Safety: acetone is highly flammable and its fumes are harmful. Ventilation, fire safety and respiratory protection are essential.
PLA
PLA does not respond to acetone.
Tetrahydrofuran (THF) smooths PLA well but is more hazardous than acetone and needs extra precautions.
Ethyl acetate, found in some nail polish removers, is less hazardous than THF and works more slowly.
The Polymaker Polysher uses isopropyl alcohol vapor on Polymaker's own PolySmooth PLA.
PETG and other materials
PETG needs aggressive solvents like dichloromethane and professional safety gear, so most makers skip it.
Specialty filaments each have their own requirements, so check before trying any solvent.
We mainly use mechanical finishing. It gives excellent results without the safety and environmental concerns of solvents.
Primer bridges plastic and paint
Filler primer
Automotive filler primer carries solids that fill small scratches and remaining layer lines. Rustoleum Filler Primer works well on PLA.
Apply several thin coats, sanding lightly between them. One thick coat drips and covers unevenly.
Repeat the cycle: prime, dry fully, sand at 400 grit, prime again. Each round fills more and smooths further.
Adhesion primer
Standard primer gives paint something to grip. PLA and PETG can be slightly slippery for some paints.
Plastic-specific primers are formulated for the plastic and bond paint better still.
Primer color
Gray is a neutral base for most colors and shows flaws that need more sanding.
White lets bright colors show true with fewer coats.
Black gives depth under metallics and richness under dark colors.
Paint turns a print into a finished piece
Brush painting
Acrylics suit detail and multi-color work. They dry fast, clean up with water and come in every color.
Dry brushing, washes and layering add depth. Tabletop miniature painters have worked out techniques that carry straight over to printed pieces.
Fine brushes handle eyes, scales and claws.
Spray painting
Spray paint covers solid colors evenly and faster than a brush. Thin coats build opacity without drips.
Metallic sprays in gold, silver, bronze and copper give dragons and fantasy pieces a convincing metal look.
Masking between colors gives two-tone effects and gradients.
Airbrushing
An airbrush gives the most control: smooth gradients, fine detail and even coverage on premium pieces.
It takes practice, including learning thinning ratios and technique.
It takes equipment: the airbrush, a compressor and ventilation.
Specialty finishes
Weathering adds age and wear with dry brushing, washes and pigment powders.
Metallic and pearl paints contain metal flake or mica for shimmer and depth.
Clear coat seals the paint, protects it from handling and sets the sheen: gloss, satin or matte.
Large pieces are printed in parts and assembled
Choosing an adhesive
Cyanoacrylate (super glue) bonds PLA and ABS fast and strong. Gel versions do not run and allow time to position.
Plastic cement welds parts by partly dissolving both surfaces. The bond is very strong but only certain plastics respond.
Epoxy fills gaps, works on every plastic and cures slowly enough for careful positioning.
Hiding the seams
Putty, such as Tamiya putty, fills the gaps between parts and sands smooth.
Careful sanding blends the seam into the surrounding surface.
Good design puts joints at natural breaks or out of sight.
Done well, an assembled piece looks like it was printed in one go.
A few advanced techniques give unusual results
Hydro dipping
A hydrographic film floats on water. Dipping the piece through it transfers a pattern, such as carbon fiber, wood grain or camouflage.
Flocking
Fine fibers on adhesive give a soft, velvet-like surface, popular on decorative pieces.
Resin coating
Epoxy resin fills layer lines completely and leaves a glossy, glass-smooth, durable surface with real depth.
Metal plating
Electroless plating deposits real metal on the plastic, using a conductive primer and specialized chemicals.
Each needs its own materials and skills.
How we decide how much finishing a piece gets
Articulated toys usually get light finishing: support marks cleaned up and joints checked for smooth movement. The layer lines add character.
Display pieces may get progressive sanding, primer and paint.
Custom orders can specify the finish, from raw print to fully painted.
Time and cost weigh in. Hours of hand finishing raise the price, and we balance that against what customers expect.
We say exactly what finish a piece has, so you know what you are getting.
Finishing is where the price difference comes from
Professional finishing takes:
- Hours of skilled sanding and painting
- Ventilation and safety equipment
- Quality primers, paints and clear coats
- Experience with which technique suits which design
- Patience
That work separates a premium collectible from a basic print. The difference between a $15 raw print and a $50 finished piece is mostly finishing labor and materials.
A smooth finish is settings, sanding, primer and paint, in that order
Whether a piece keeps its layer lines or ends up glass-smooth and painted, the finish decides how it reads. We finish each piece to the level that suits its purpose and tell you which it is.
Keep exploring
The finish on a flexi is only half the story. Our care guide explains how to keep joints moving and colors bright, and the best sellers collection shows what a well-finished print looks like.