Gluing 3D printed parts and hiding the seams
A split print is only as good as its seams. The right glue for your material, careful clamping and a patient filler and primer routine make the joints disappear under paint.
1. Prepare the surfaces
- Dry-fit every joint first: pins should seat fully and the seam should close without force.
- Remove brim marks and elephant foot by rubbing the cut faces on sandpaper laid on a flat surface.
- Scuff the mating faces with 120–180 grit so the glue can grip.
- Degrease with isopropyl alcohol and let it dry.
- Resin parts: wash and fully post-cure them first; a tacky surface weakens any bond.
- Plan the order: glue sub-assemblies first (one row, one limb), then join them, so small errors do not add up.
Part numbers engraved on the cut faces, like the ones the SplitSTL editor adds, save a lot of guessing at this stage.
2. Choose a glue for your material
| Method | Sets in | Strengths | Weak points |
|---|---|---|---|
| CA (super glue), thin or gel | Seconds | Fast, strong on tight joints; an activator speeds it up | Brittle, fills gaps poorly, white haze around the joint |
| Two-part epoxy | 5 min to 24 h depending on type | Fills gaps, tough, leaves time to align | Must be mixed accurately, slower |
| Plastic welding (3D pen or soldering iron with filament) | As soon as it cools | Fuses the same plastic into one piece | Leaves a bead to sand; heat can warp thin walls |
| Solvent welding (ABS/ASA with acetone) | Minutes | Melts the parts together; ABS slurry fills gaps | ABS and ASA only; ventilate |
- PLA: CA for small parts, epoxy for large ones, and welding works well. Acetone does not dissolve PLA.
- PETG: resists most solvents; scuff it well and use epoxy, or weld it.
- ABS and ASA: acetone or ABS slurry (ABS dissolved in acetone) gives a true weld.
- Resin: CA, or liquid resin cured with a UV lamp, which also fills small gaps.
3. Plastic welding, step by step
- Use filament of the same material as the parts, and a spare soldering iron tip: it will get coated in plastic.
- Tack the joint at a few points first by melting a short piece of filament into the seam.
- Then run the tip along the seam, pushing filament in and stirring it into both edges, so the plastic fuses instead of sitting on top.
- Set the iron near the material’s printing temperature: too hot and PLA bubbles and thin walls distort. Work in short sections and let them cool.
- Weld from the inside where you can, so the outside only needs light filling.
- Ventilate, because molten plastic gives off fumes.
4. Clamp and align while it sets
- Rubber bands, painter’s tape, spring clamps or printed clamps all work: the pins or pegs align, the clamp only has to hold.
- On a large part, put epoxy on most of the face and a few dots of CA gel near the edges, then close the joint: the CA grabs in seconds and holds the alignment while the epoxy cures.
- Do not over-tighten epoxy joints: squeezing all the glue out starves the joint.
- Wipe epoxy squeeze-out with isopropyl alcohol before it sets; cured glue is much harder to sand than plastic.
- Wait for the full cure time stated on the glue before sanding.
5. Fill the seam
| Filler | Best for | Good to know |
|---|---|---|
| CA + baking soda | Small gaps, fast | Sets instantly but sands harder than the plastic around it |
| Acrylic spot putty | Scratches and gaps under 1 mm | Shrinks slightly, may need two passes |
| Two-part polyester body filler | Deep gaps, reshaping | Sands well; work with ventilation |
| Epoxy putty | Rebuilding edges and details | Smooth it with water before it cures |
| UV resin | Resin prints | Cures in seconds under a UV lamp |
Apply it slightly proud of the surface and sand it flush once cured.
6. Sand, prime and paint
- Level the filler with 120–180 grit on a sanding block, so flat areas stay flat.
- Refine with 240–320 grit.
- Spray two or three light coats of filler primer.
- Wet-sand with 400, then 600 grit: water keeps PLA from heating up and clogging the paper. Avoid fast rotary tools, which melt it.
- Repeat primer and sanding until the seam disappears under raking light.
- Paint in thin coats, then seal with a matt, satin or gloss clear coat.
Spray at mild temperatures and low humidity, and test any new paint on a scrap print first.
Frequently asked questions
What is the best glue for PLA?
CA for small parts and tacking, two-part epoxy for large or load-bearing joints. Welding with PLA filament is the strongest option if you are happy to sand the bead.
How do I glue PETG?
Scuff both faces, degrease them with isopropyl alcohol and use epoxy. Welding with PETG filament works too.
How do I make a seam invisible?
Fill slightly proud, sand flat with a block, then alternate filler primer and wet sanding until no line shows under low, raking light.
Can I glue resin prints?
Yes, with CA, or with a little liquid resin cured by a UV lamp, which bonds and fills at the same time.