Remove Stress Marks & Separate Gundam Parts Cleanly

This guide shows you how to remove the small white or discolored marks that appear when you cut a Gundam kit part from its runner, and how to separate parts without leaving nubs, scratches, or stress fractures. You’ll learn the correct order of cutting, sanding, and polishing so your model looks like it was injection-molded that way.

What You’ll Need

• Side cutters (nippers) – preferably single-bladed for a flush cut
• Hobby knife or scalpel with a sharp #11 blade
• Sanding sticks or sandpaper (grits: 400, 600, 800, 1000, 1200, 1500, 2000)
• Polishing cloth or a soft microfiber cloth
• Optional: glass file (like the ones sold for nail care, but a dedicated hobby glass file works best)
• Optional: plastic cement or thin liquid glue (for parts that snap together but need extra strength)
• Optional: masking tape (to protect adjacent surfaces while sanding)

Step-by-Step

1. Cut the part from the runner with a margin. Do not cut flush against the part. Leave about 1–2 mm of the nub attached. This prevents the cutter’s pressure from stressing the plastic at the connection point, which is the main cause of stress marks.
2. Make a second, closer cut. Flip the part so you can see the nub clearly. Position the flat side of the nipper against the part surface and the angled side toward the nub. Gently squeeze to remove most of the remaining nub, leaving just a tiny sliver (0.3–0.5 mm). Do not force it – if it feels tough, make a third cut instead of pushing harder.
3. Scrape the remaining nub with a hobby knife. Hold the blade perpendicular to the part surface and lightly scrape across the nub, not into it. Use short, gentle strokes, always moving the blade away from your fingers. This shaves the nub down to near-zero without gouging the plastic.
4. Sand the area with progressively finer grits. Start with 400-grit sandpaper or a sanding stick. Sand in one direction (not back-and-forth) with light pressure. Wipe away dust with your finger or a cloth, then move to 600, 800, 1000, 1200, 1500, and finally 2000-grit. Each step removes the scratches from the previous grit.
5. Check for stress marks under good light. Hold the part at an angle to a light source. Stress marks appear as white or cloudy areas. If you see any, go back to the 400-grit and sand a little more, then work up through the grits again.
6. Polish the area. Use a polishing cloth or a soft microfiber cloth to buff the sanded spot. Rub in a circular motion for about 30 seconds. This restores the original sheen and blends the sanded area with the surrounding plastic.
7. Separate parts that are stuck or fused. If two parts are already attached (e.g., a ball joint that’s too tight), do not twist or pull hard. Instead, use the hobby knife to gently score the seam line, then apply a tiny drop of plastic cement to the joint and wiggle it free. For parts that are simply snapped together, use a flat spatula or a part separator tool to gently pry them apart, starting at a corner and working around the edge.
8. Clean the part before assembly. Wipe the part with a dry cloth to remove sanding dust. If you used any polish or cement, let it dry completely before fitting the part.

Common Mistakes

• Cutting too close on the first snip. This puts direct pressure on the part’s surface, causing a white stress mark that is hard to remove.
• Using dull nippers or blades. A dull edge crushes the plastic instead of cutting it, creating more stress marks and rough nubs.
• Skipping grits. Jumping from 400 to 1000 leaves deep scratches that the finer grit cannot fully remove.
• Sanding too hard. Heavy pressure creates heat and friction, which can melt the plastic and cause a shiny, deformed spot.
• Forgetting to check the part from multiple angles. Stress marks are often visible only from one side.

Tips

• Work in a well-lit area. A desk lamp with a bright white bulb helps you see stress marks and sanding progress.
• For colored plastic, use a glass file instead of sandpaper. Glass files remove material quickly and leave a smoother finish, reducing the number of grits needed.
• If you’re painting the kit, you can skip the fine polishing (1200+ grit) because primer and paint will cover minor scratches.
• Use a cutting mat to protect your work surface and your blades.
• When sanding curved surfaces, wrap the sandpaper around a cotton swab or a small dowel to follow the contour.
• Keep a separate pair of nippers for cutting metal or thick plastic – don’t use your good Gundam nippers for anything else.

Troubleshooting

• Stress mark still visible after sanding. The mark may be deeper than you thought. Sand a bit more with 400-grit, then redo the whole sequence. If the mark is in a recessed area, use a small piece of sandpaper folded into a point.
• Part has a shiny, melted spot. You sanded too fast or with too much pressure. Let the part cool, then lightly sand with 800-grit and work up again, using very light pressure.
• Nub broke off leaving a gouge. This happens when you cut too close or twist the nippers. Fill the gouge with a drop of plastic cement and let it dry, then sand flat. Alternatively, use a small amount of putty for plastic models.
• Parts won’t separate without force. Apply a drop of liquid cement to the seam – it will soften the plastic slightly and allow the parts to separate. Do not use excessive force, or you may snap a peg or tab.
• White mark appears after assembly. This is often caused by the part being stressed during fitting. Apply a tiny bit of plastic cement to the joint and let it dry – the cement melts the surface and can eliminate the mark.

Frequently Asked Questions

Why do stress marks appear in the first place?

Stress marks are micro-cracks in the plastic caused by compression or bending. When you cut a part from the runner, the nipper crushes the plastic at the cut point, creating these tiny fractures that scatter light and appear white.

Can I remove stress marks without sanding?

Yes, in some cases. Rubbing the mark with your fingernail or a soft cloth can sometimes reduce it, but it won’t fully remove it. For a clean result, sanding is the most reliable method.

Is a glass file better than sandpaper?

For most hobbyists, a glass file is faster and leaves a smoother finish, but it can be aggressive on thin parts. Sandpaper gives you more control and is better for curved or delicate areas.

Do I need to polish after sanding?

Polishing is recommended if you want a glossy, seamless look. It’s not necessary if you’re going to paint the part, because primer will cover the sanded surface.

What if I accidentally cut a part too short?

If you cut off a peg or a tab, you can pin it with a small piece of wire or use plastic cement to reattach it. For a clean fix, drill a tiny hole in both parts and insert a piece of paperclip, then glue.

Can I use nail clippers instead of hobby nippers?

Nail clippers work in a pinch, but they crush the plastic more than they cut, causing more stress marks. If you’re serious about Gundam building, invest in a pair of single-bladed hobby nippers.

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