How to remove mould lines from kits and minis

What a mould line is

Plastic, resin and metal parts are cast in moulds made of two or more pieces. Where those pieces meet, a hairline ridge of material is left on the part. On a figure it usually runs along the sides of the arms, legs and head; on a kit part it follows the outer edge.

Mould lines are easy to miss on bare grey plastic and very obvious once the model is primed or painted, which is why they belong in the prep stage, before glue or paint goes anywhere near them.

Find them first

Turn every part under a good light before you start, and look along the edges rather than straight at the face. Goonhammer's beginner guide makes the point that it is all too easy to miss a line and only find it when the model is fully painted, so inspect each component from every angle.

Common bench tip: running a fingernail along an edge will often catch a line you cannot see.

Scrape, do not slice

The standard tool is the back (the blunt spine) of a hobby knife blade, or a dedicated mould line scraper. Hold the blade roughly upright to the surface and drag it along the line with light pressure, so it shaves rather than cuts. Several light passes that each lift a fine curl give a smoother finish than one heavy one.

Wargaming Tradecraft describes the two ways to use a knife: scraping, with the edge angled away from the direction of travel so it levels the surface, and slicing, with the edge angled into the cut so it removes material. Scraping is the safer default; save slicing for leftover sprue nubs, and always cut away from yourself.

Rotate the part rather than your wrist, so the blade stays at the same angle as you follow a curve.

Files and sanding sticks

Needle files reach places a straight blade cannot: a flat file for flat panels, a half-round file for limbs, a round file inside folds. Files are also the safer choice if a younger builder is helping, though they leave a rougher surface than scraping.

Finish with a fine sanding stick or sponge. Mini Paint Lab suggests starting at around 400 to 600 grit to level a stubborn line and moving to 800 or 1000 grit to remove the scratches.

Common mistakes

Using the sharp edge of the blade. It digs in, cuts rivets and small detail off, and flattens curved surfaces.

Clipping flush to the part in one snip. Leave a small stub and shave it down; a single flush cut can tear a divot that is harder to repair than a stub is to remove.

Common bench practice: do not forget resin parts need washing as well as cleaning up. Mould-release residue can stop paint and glue keying.