Seams and finishing
This guide covers seams between assembled parts, not the Z seam of a print made in one piece. The filling and priming steps are mainly intended for decorative models that will be painted.
1. Decide whether the model will be painted
Before filling, choose the desired finish. On a multicolour model kept raw, start by improving the fit and avoiding glue marks. A filler of another colour does not automatically become invisible after sanding.
For your next splits, try to place the seams in a fold, at a colour boundary or between two elements of the model, rather than in the middle of a large smooth surface.
Without paint, the realistic goal is often a discreet seam, not its complete disappearance.
2. Check the joint before covering it
Check the alignment and wait for the glue to cure. If the parts still move, fix their attachment first. Do not rely on a finishing filler to hold an unstable assembly.
A slight hollow and a large offset between two parts do not call for the same correction. Before filling a marked defect, check that the parts are in their intended position.
For this step, see “How to assemble a model printed in several 3D parts”.
3. Fill only the area that needs it
Choose a filler compatible with the plastic and suited to the depth of the defect. A modelling putty may do for a small joint; some two-part epoxy fillers can make up a larger gap. Check the instructions and test on an offcut.
Work locally and respect the recommended thickness. For a two-part product, follow the intended proportions: adding more hardener is not an adjustment method.
Let cure according to the instructions before sanding.
4. Sand progressively without erasing the details
Sand off the excess, then move to a finer abrasive to blend the scratches. The progression depends on the filler, the plastic and the surface finish: use the manufacturer’s guidance rather than a single grit for the whole model.
On a flat surface, use a small sanding block. On a rounded shape, follow the curve and check the result often. Protect the edges, engravings and nearby textures.
Stop regularly to remove the dust and examine the area.
5. Apply a control primer
On a model meant to be painted, apply a thin coat of primer compatible with the plastic and the filler. It helps reveal the defects still present.
After drying, look at the seam with light coming from the side. If a hollow or a bump is still visible, rework the filling or the sanding locally.
Avoid thick coats: they may soften the details without properly correcting a significant defect.
6. Paint when the surface is ready
Remove the dust and respect the drying and recoating times of the products. Then move on to painting with a system compatible with your primer.
Do not rely on an extra coat of paint to hide a seam that is still raised. Correct its shape before finishing, then check the result from several angles.
Precautions
Avoid inhaling sanding dust. For fillers, primers and paints, follow the safety data sheet: ventilation and protection of the eyes, skin and airways suited to the product and its application.
Key points
A stable, well-aligned joint is treated before painting: local filling, progressive sanding, then a check with primer. On a model kept in the colours of the filaments, focus above all on the fit and on well-placed seams.