Cutting & Assembly

White stress marks and nub marks after cutting parts

Pale, cloudy patches where a part was cut from the runner. They come from crushed plastic, and a two-cut method plus light sanding prevents most of them.

Quick answer

White stress marks appear when a nipper crushes or bends the plastic instead of slicing it. Cut a few millimeters away from the part first, then make a second close cut with a sharp single-blade nipper and clean the stub with a blade and fine sanding. Marks that remain can often be rubbed or sanded out on bare plastic, and primer, paint or a matt top coat hides them.

Time1-3 minutes per part
DifficultyBeginner

Safety first

Cut away from your fingers and keep the blade covered when not in use.
Wear eye protection if cutting hard or brittle runners: small pieces can fly.

Is this your problem?

The mark is a pale patch right at the cut pointIt is a stress mark from cutting: this page.
There is a small raised bump left at the cut pointThat is a nub, not a stress mark: shave and sand it as in steps 3-4.
The plastic has a thin crack or split, not just whiteningIt is a crack: glue it with thin plastic cement or CA and ask us if it is on a load-bearing joint.
A white haze appears around a glued jointThat is super glue frosting, not cutting stress: see Fogged clear parts.

You'll see

A pale, cloudy or whitish patch on the plastic right where the gate (the little bridge to the runner) was. It shows most on dark, red, blue or glossy plastic and on unpainted Gunpla.

Likely causes

Cutting flush in one biteThe blade has to push through a thick gate right at the part, so the plastic next to the cut is squeezed and bent. Bent plastic scatters light and looks white.
A blunt or thick double-blade nipperTwo thick blades meeting in the middle wedge the plastic apart instead of slicing it, which stresses both sides of the cut.
Twisting, pulling or bending the part off the runnerAny bending beyond the plastic's elastic limit leaves a permanently stressed, whitened zone.
Cold or very brittle plastic (some clear, metallic and soft-colored plastics)Brittle plastic stresses and whitens more easily; clear and plated parts show it most.

What you need

Sharp single-blade nipper (or any sharp nipper for the first cut)
Hobby knife or pen knife
Sanding sticks or sponges, roughly #400 to #1000 and finer
Optional: polishing grit (#2000+) or a matt top coat for unpainted kits

How to fix it

First cut: snip the gate 2-3 mm away from the part, so the stress lands on the runner and the stub, not on the part.
Second cut: with a sharp single-blade nipper, cut the stub close to the part with the flat side of the blade facing the part. Leave a hair of material rather than biting into the surface.
Shave what is left with a sharp blade in small passes, moving the blade away from the part and keeping it almost flat to the surface.
Sand the spot flush with a sanding stick, working from a medium grit (around #400-#600) up to finer grits (#800-#1000 and above). Sand the gate area only, with light even strokes.
If the part stays unpainted, keep going to very fine grits (#2000 and up) or a polishing sponge to bring the sheen back, or plan a matt top coat, which hides most remaining whitening.
If you will prime and paint, stop at a smooth surface: primer and paint cover a faint mark completely.

What to avoid

Cutting flush in a single bite with a thick double-blade nipper.
Twisting or pulling a part off the runner.
Sanding hard on one spot, which leaves a visible dip.
Using a single-blade nipper on thick runners or metal: it chips the blade.

Common wrong fixes

Heating the mark with a lighter or heat gun: heat can soften, warp or melt thin parts. Not recommended.
Coloring the mark with a marker on unpainted plastic: it usually looks worse under light.

Not worth fixing, or time to ask us

A light white mark on a part you will paint is not worth chasing. Ask us if the plastic is cracked rather than just pale, or if marks appear even with a sharp single-blade nipper.

Send us a photo Read the full step-by-step guide

By situation or hobby

Unpainted Gunpla (straight build)

Prevention matters most here because nothing will cover the mark. Two-cut method, then sand and finish with a matt top coat; many builders find light rubbing with a fingernail or the back of a smooth tool reduces small marks on flush cuts.

Painted models (armor, aircraft, cars, ships)

A faint mark is cosmetic: primer hides it. Spend the effort on a flush, smooth surface instead.

Clear parts and plated (chrome) parts

These show stress worst and cannot be sanded without losing clarity or plating. Cut further away, use your sharpest nipper and, on clear parts, polish with very fine grits afterwards.

Gates on curved or visible faces

Use a sanding sponge rather than a flat stick so you follow the curve and do not leave a flat spot.

Already ruined?

If a mark is already there on bare plastic: sand it with fine grits until the white disappears, then polish or top-coat. On a painted model, sand lightly, re-prime that spot and repaint. If sanding made a dip, fill it with a thin layer of putty or gap-filling CA, cure, sand and re-prime.

Next time

Always cut twice: away from the part first, close second.
Keep one sharp single-blade nipper only for the second cut and never cut metal or thick runners with it.
Hold the runner, not the part, while cutting.
Cut with the flat face of the blade towards the part.
Warm the room if the plastic is cold; brittle plastic whitens more.

Frequently asked questions

Do stress marks go away on their own?

No. The whitened plastic stays stressed. They can be reduced by sanding and polishing, and hidden with primer, paint or a matt top coat.

Can you remove stress marks with heat?

Some people try it, but heat can warp or melt thin parts and plated plastic. Sanding, polishing or a top coat is safer.

Do single-blade nippers stop stress marks?

They reduce them a lot because they slice instead of crush, but technique still matters: always make the first cut away from the part.

What grit should I sand nub marks with?

Start around #400-#600 to level the nub, then go finer (#800-#1000 and up). For bare plastic, finish at #2000 or with a polishing sponge.

Does a top coat hide stress marks?

A matt top coat hides most light whitening on unpainted Gunpla. Gloss top coats hide less.

What is the difference between a nub mark and a stress mark?

A nub is leftover plastic sticking out at the cut. A stress mark is whitened plastic. You trim a nub; you sand or cover a stress mark.

Jargon explained

Gate: The small bridge of plastic that joins a part to the runner (sprue).
Nub: The stub left on the part after cutting the gate.
Top coat: A clear varnish sprayed over the finished model, usually matt, semi-gloss or gloss.

Tools that can help

NightWorks NW-MP02 Ultra-Thin Single Edge Nipper $36.95
Why it helps: A single sharpened blade cuts against a flat anvil, so gates are sliced rather than crushed and leave minimal stress marks.

How to use it
Save it for the second, close cut right next to the part.
Pen-type Trimming Knife For Gundam And Scale Modeling $22.95
Why it helps: A fine pen-style knife for shaving what is left of the gate in small, controlled passes.

How to use it
Shave the stub gently with the blade moving away from the part.
Hobby Mio HMD Double-Sided Sanding Sticks from $10.95
Why it helps: Each stick has two grits, so you can go from levelling to smoothing without changing tools.

How to use it
Level with the coarser face, then finish with the finer one.
Stedi 10pcs/box #400~15000 3mm Sanding Sponges from $10.95
Why it helps: Flexible sponges from #400 up to very fine grits that follow curves where flat sticks leave flat spots.

How to use it
Use the finest grits last on bare plastic to bring the sheen back.