A mould line is the faint raised seam left where the two halves of an injection mould met. Every plastic and resin miniature has them. They run down arms, along weapon barrels, over shoulder pads and across the tops of heads, and they are almost invisible on bare grey plastic.
Then you prime the model and they appear everywhere. This guide covers how to find them, how to remove them without destroying the detail underneath, and what to do when you spot one too late.
Why mould lines only show up after priming
Bare plastic is a single flat colour, so your eye reads shape but not fine surface geometry. Primer changes that. It gives the whole model one matt tone and a slight texture, and suddenly every ridge catches light differently from the surface around it.
Washes make it worse. A shade flows along the base of a raised line and settles there, drawing a dark pencil stroke down the length of the seam. Drybrushing makes it worse again, because a drybrush catches raised edges preferentially and paints a bright line directly on top of the mould line.
So a seam you could barely see on the sprue ends up outlined in both dark and light by the time the model is finished. Ten minutes of preparation prevents all of it.
Tools
- Mould line scraper. A blunt-edged tool designed to shave the seam without cutting into the surface. The most forgiving option and the one to buy if you buy anything.
- Hobby knife. Used with the blade held perpendicular and dragged, not pushed. A fresh blade is essential; a dull one skips and gouges.
- Fine sanding sticks or sponges. Best for finishing after scraping, and for curved surfaces where a blade cannot sit flat. Very fine grits only.
- Small needle files. Useful on flat armour panels and thick resin. Too aggressive for fine detail.
- Good directional lighting. A single angled light source is the tool that actually finds the lines. Overhead room lighting hides them.
Blade safety
Hobby knives are sharper than most people expect and the injuries are usually to the hand holding the model. Cut and scrape away from your fingers, never towards them. Keep the model supported on a surface rather than held in mid-air. Change blades when they stop cutting cleanly, because a dull blade needs more force and slips more readily. Retract or cap the blade the moment you put it down.
Step by step
- Inspect under directional light. Hold the component at an angle to a single lamp and rotate it slowly. Mould lines appear as a thin bright or dark edge that runs continuously across surfaces regardless of the sculpted detail. Running a fingernail across the surface also finds them by feel.
- Identify seams versus sculpted detail. This is the step that matters most. A mould line runs in one continuous plane around the model, cutting straight across armour, cloth and skin without regard for what it crosses. A sculpted panel line or fold follows the design and stops where the design stops. If a raised line respects the sculpt, leave it alone.
- Support the component securely. Rest it on a cutting mat or hold it against a block. Working on a part held loosely in the air is how blades slip.
- Scrape rather than cut. Hold the blade or scraper roughly perpendicular to the surface and draw it along the seam with light pressure. You want to shave the ridge off, not slice into the plastic. Several light passes beat one firm one.
- Work gradually around curves. On a rounded surface, keep the tool moving and rotate the part as you go so the edge stays tangent to the curve. Stopping mid-stroke on a curve creates a flat spot.
- Finish with fine abrasive. A light pass with a fine sanding stick removes the faint scraping marks and blends the area back into the surrounding surface.
- Inspect again from multiple angles. Rotate the part under the light again. Seams you missed on the first pass are usually on the underside of arms, the back of legs and the top of the head.
- Clean off debris before priming. Scraping produces fine plastic dust that sits in recesses and stops primer adhering. Brush it off with a soft dry brush, or rinse and dry the part.
Mould lines on armour
The easiest case. Flat and gently curved armour panels give the scraper a stable surface to run along, and there is usually no fine texture to damage. Scrape, then sand lightly, then check under the light.
The one thing to watch is edges. It is easy to round off a crisp armour edge while chasing a seam that runs along it. Work along the flat and stop short of the edge, then deal with the edge itself using a very light sanding pass.
Mould lines on organic surfaces
Fur, cloth folds, hair, faces and scaled surfaces are the difficult cases, because scraping removes the sculpted texture along with the seam and leaves a smooth stripe that looks worse than the original line.
Two approaches work. The first is to scrape very lightly with the tip of the blade, following the contour of the texture rather than running straight through it, and accepting that you will reduce rather than eliminate the line. The second is to remove the seam properly and then restore texture over it, either by stippling with a knife tip or by breaking the smooth area up with paint later.
On faces in particular, be conservative. A slightly visible seam on a cheek is far less noticeable at tabletop distance than a flattened cheekbone.
Removing mould lines from tiny weapons
Barrels, blades, antennae and thin poles are fragile and hard to hold. A few things help:
- Do this work while the part is still attached to the sprue where possible. The sprue is a handle and a support.
- Support the part along its length rather than gripping one end, so it cannot flex and snap.
- On a cylindrical barrel, rotate the part against a stationary blade rather than dragging the blade along the part.
- For very thin parts, skip the blade entirely and use a fine sanding sponge, which applies less concentrated force.
What about already-assembled miniatures?
Access is the problem, not technique. A seam running down the inside of an arm becomes unreachable once the arm is glued to the torso.
Do what you can with the scraper tip and a fine sanding stick folded into a narrow strip. Where a seam genuinely cannot be reached, leave it. A partially cleaned model is a normal outcome and nobody will notice a line in an armpit.
The wider lesson is to check for mould lines during assembly rather than after it. Cleaning parts on the sprue, or at least before gluing, takes less time and produces a better result. Our guide to painting your first miniature covers where preparation fits into the overall sequence.
Can you remove mould lines after priming?
Yes, and it is a common rescue. Scrape or sand the seam as normal. You will cut through the primer and expose bare plastic along the line, which then needs re-priming before you paint. A small brush-on primer touch-up is usually enough; there is no need to re-prime the whole model.
After base coats have gone on it gets harder, because you will also be removing paint and creating a visible step at the edge of the repair. It is still possible on a flat panel. On detailed areas it is often better to leave the line and accept it.
Common mistakes
- Cutting instead of scraping. Pushing a blade along a seam digs a groove. Dragging it perpendicular shaves the ridge off.
- Removing sculpted detail. Panel lines, seams in the armour design and fabric folds are meant to be there. Check whether the line respects the sculpt before you touch it.
- Working under flat overhead light. You cannot remove what you cannot see. One angled lamp finds ten times as many seams.
- Using a dull blade. More force, less control, more slips.
- Sanding too coarsely. A coarse grit leaves scratches that show up under primer just as clearly as the seam did.
- Priming over plastic dust. Causes patchy adhesion and a gritty finish.
- Chasing perfection. Some seams sit in places that cannot be cleaned without damage. Leave those.
Frequently asked questions
Do I really need to remove mould lines?
For a model you intend to paint properly, yes. It is the cheapest quality improvement available. For rank-and-file models painted quickly, doing the visible faces and skipping the undersides is a reasonable compromise.
How long should this take per model?
Five to ten minutes for standard infantry once you know where to look, less as you get quicker. A vehicle with large flat panels can take longer simply because there is more surface.
Are resin miniatures different?
Resin is more brittle and tends to have flash, thin webs of excess material, as well as seams. Flash usually snaps or trims away cleanly. Resin also sands more readily than plastic, so use a lighter touch and finer grit than you would on a plastic kit.
What if I damage the model?
A small gouge can be filled with a thin smear of modelling putty, left to cure and sanded flush. A flattened texture can often be disguised with paint. Neither is a disaster.
Should I wash the model afterwards?
A rinse in warm soapy water and a thorough dry removes both scraping dust and handling grease, and it costs nothing. It matters more for resin than for plastic. Once it is fully dry, it is ready for primer. Our guide to priming miniatures properly covers the next stage.
If you have already painted a model and spotted a seam you missed, our guide to fixing painting mistakes covers what is recoverable and what is not.
Independent guide published by Blubbercove Ltd/WarSplay. This article is not affiliated with, sponsored by, endorsed by, authorised by or licensed by Games Workshop Limited. Warhammer 40,000 is a trade mark of Games Workshop Limited.