How do I get rid of the seam lines on a plastic model?

A visible seam is the single most obvious sign of a rushed build, and it is almost never noticed on bare plastic because grey styrene hides everything. It appears the moment primer goes on, by which point most people have already painted. Doing this properly means accepting that the check happens under primer and that you may go round twice.

Safety

What you'll need

• Thin liquid cement
• Sanding sticks or wet-and-dry paper in a range of grits
• Model filler — a putty type and a liquid type
• A scriber and flexible tape for a guide (optional)
• Primer, in a colour you can read

Step by step

1. Deal with mould seams before assembly, not after.The fine raised ridge running along a part where the two halves of the mould met is different from a joint seam. Scrape it off with the back of a blade held at ninety degrees to the surface, drawn along the ridge. Scraping beats sanding here because it does not round the shape.
2. Weld the joint rather than glue it.Hold the parts closed, run thin cement into the seam and squeeze until a bead of softened plastic rises out of the join. That bead is your filler and it is the same material as the part.
3. Wait longer than feels necessary.A welded seam is soft well below the surface for hours. Sand it too early and the softened plastic dishes as it finishes shrinking, leaving a shallow valley that only shows under primer. Overnight is not excessive on a heavy joint.
4. Sand the bead flush across the seam.Work the abrasive at right angles to the joint line, not along it, and use a rigid sanding stick rather than paper on a fingertip. A finger follows the contour and dishes the softer side.
5. Fill only what the weld did not close.Putty filler for a real gap, applied slightly proud because every filler shrinks as its solvent leaves. Liquid or thin filler for scratches and a sink mark. A flush application ends up sunk and you fill twice.
6. Step through the grits.Start only as coarse as the job needs, then work finer in steps, each step removing the scratches of the last. Skipping from coarse to very fine leaves deep scratches polished at the surface, which primer finds immediately.
7. Re-scribe lost detail.Sanding removes panel lines and rivets along with the seam. Lay flexible tape as a guide beside where the line was, and draw a scriber along it in light repeated passes, deepening gradually rather than trying to cut it in one.
8. Prime and inspect under a raking light.One thin coat of primer in a mid tone, then look along the surface with a light source almost parallel to it. A shadow along the seam means it is still there. This is the only reliable check.
9. Go round again if it shows.Local sanding, local filler, local primer. Accepting a seam at this point means it will be the first thing anyone sees on the finished model.

Tips

• Sanding wet with wet-and-dry paper cuts the dust, clogs less and gives a finer finish for the same grit. A drop of washing-up liquid in the water helps.
• A sanding sponge follows a curve without dishing it, where a rigid stick will flatten it. Match the tool to the shape.
• If a seam runs through raised detail, cut the detail off first with a blade, deal with the seam, then replace the detail with plastic card or stretched sprue. Trying to sand around it never works.

Common mistakes

• Filling before welding — Filler is softer than the plastic either side, so sanding a filled joint cuts the filler faster than the surrounding surface and leaves a groove. Welded plastic sands at the same rate as the part.
• Judging the seam on bare grey plastic — Unpainted styrene shows almost no surface variation. Every seam looks gone. Primer is a measuring instrument as much as a coating.
• Sanding along the seam — The abrasive rides along the softer welded line and cuts it deeper than the surrounding plastic, so a seam you were removing becomes a groove you now have to fill.

If it doesn't work

Questions people ask