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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallTo separate a Gundam part without a white stress mark, do not make one flush cut against the part. Use plastic model nippers to cut the runner a short distance away, leaving a small nub. Reduce that nub gradually with a file or sanding stick, then refine the surface with progressively finer abrasive. Bandai’s example sequence is 400–600 grit followed by 1000-grit or finer polishing.
This approach protects the finished surface while giving you material to remove in controlled stages. A mark that is already present may not disappear completely through sanding, so treat cosmetic touch-up as optional rather than a guaranteed repair.
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What you are removing: runner, gate and nub
Gundam kits are molded on runners (also called sprues). Each finished piece is joined to the runner by a small molded connector, commonly called a gate. After you cut the connector, the short projection left on the part is the nub.
The goal is not to force the first cut to produce a finished edge. The goal is to separate the part while leaving enough sacrificial plastic for a second, controlled cleanup. Cutting directly at the part can load the surrounding plastic and create a pale or white area. Bandai describes the cause plainly in Tips To Improve Your Gunpla: “When you cut too close to the required piece, you could leave white marks which are caused by stress from the plastic.” Bandai’s official Gunpla tips use this warning as the reason to leave material for later cleanup.
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Tools that make the process predictable
| Tool | Job | How to use it safely |
|---|---|---|
| Plastic model nippers (side cutters) | Separate the part from the runner | Make the first cut away from the part so a nub remains for cleanup. |
| File or sanding stick | Reduce the remaining nub | Work slowly and stop before you flatten adjacent edges or molded detail. |
| Sandpaper | Refine and polish the area | Use a coarser stage for leveling and a finer stage for the final surface. |
| Hobby knife | Optional localized trimming | Use only when you can control the blade; it is not required for the core filing-and-sanding method. |
| Marker | Optional cosmetic touch-up | Choose a compatible color and finish; a marker does not reverse stress in the plastic. |
Bandai’s official beginner guidance lists plastic nippers for cutting parts from sprues. Its tool catalog also lists a Model Sanding Stick Set and a Model Separator; the catalog establishes those product categories, but a separator is not required to cut parts from runners. See the Bandai Hobby beginner guide and Bandai Hobby tools catalog.
Choosing and setting up supplies
A basic setup does not require a large tool collection: plastic model nippers, a file or sanding stick, and abrasive in the 400–600 and 1000-or-finer ranges cover Bandai’s example process. A hobby knife is optional, not a prerequisite. If you are shopping online, “plastic model nippers” is the useful category phrase; model sanding sticks and sandpaper are the other relevant supplies.
Prices, stock and brand performance vary by retailer, and no universal price is established here. Choose a tool you can hold steadily and replace abrasive when it becomes loaded. Keep the part, runner and manual together so a tiny piece is not lost while you change tools. Good lighting matters more than adding another specialized cutter: reflected light reveals whether a patch is level and whether a molded line is beginning to soften.
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The low-risk two-cut method
1. Identify the gate and inspect the surrounding surface
Before bringing in the nippers, find the connector between the runner and the part. Check whether the gate is next to a sharp edge, a recessed line, a curved armor panel or a visible exterior face. The more exposed or detailed the area, the more valuable a small leftover nub becomes: it gives you room to level the surface without forcing the initial cut to do all the work.
Read the kit manual before cutting. Manuals can show the intended part orientation and assembly order, which helps you distinguish a gate from a molded detail that must remain. If the plastic or gate looks unusual, keep a piece of unused runner for a test before trying a finishing product on the visible part.
2. Make the first cut away from the part
Position the nippers on the runner side of the gate, leaving a visible amount of connector attached to the part. Close the cutters cleanly rather than twisting the runner. The first cut is only a separation cut; it is not the final trim.
Bandai’s assembly guidance likewise describes cutting parts from the runner and then dealing with rough edges. The exact distance to leave is not given as a universal measurement, so judge it by the gate and the part’s geometry instead of treating a particular number as mandatory.
3. Remove the runner section from your workspace
Once the part is free, put the larger runner aside and inspect the remaining nub under good light. Rotate the part so you can see the nub’s base, its height and the nearby details. This short inspection prevents you from filing the wrong area or mistaking a designed ridge for leftover plastic.
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4. Reduce the nub in stages
Use a file or sanding stick to bring the nub down gradually. Keep the abrasive on the nub and make short, controlled passes. Change the direction of your passes as needed to keep the surface even, but do not press hard enough to dig a hollow into the panel.
Bandai’s example is to file the nub first, then sand with 400–600 grit, followed by polishing with 1000-grit or finer abrasive. These values are a manufacturer example, not a promise that every plastic color, gate shape or finish will respond identically. Stop each stage when the scratches from the previous stage have been reduced, rather than continuing until surrounding detail starts to disappear.
5. Blend the surface with 400–600 grit
Use 400–600 grit sandpaper to refine the filed area. Keep the abrasive aligned with the local plane of the part. On a flat panel, support the paper on a firm surface or sanding stick; on a curved section, use a backing that follows the curve so you do not create a flat spot.
Sand only enough to level the nub and the visible transition around it. Sanding removes surface material. If you keep working after the blemish has leveled, you can soften a corner, widen a recess or erase a small molded line. Bandai’s instructions support gradual cleanup but do not specify a quantified material-removal limit, so visual inspection should determine when to stop.
6. Polish with 1000 grit or finer
Move to 1000-grit or finer abrasive for the final refinement. This stage reduces the coarser sanding scratches and produces a more uniform bare-plastic surface. Use light pressure and clean abrasive; a loaded sanding surface can make it harder to see whether you are polishing or still removing substantial material.
Why a flush first cut can turn white
A flush cut concentrates the cutting action at the edge of the finished part. The resulting stress can show as a white area in the plastic, even when little or no nub appears to remain. That is why the two-cut method deliberately leaves a connector to remove later.
Do not assume every white mark is simply excess nub. If the whitening extends beyond the cut area, sanding may require removing more of the surrounding surface than you intended. The official guidance explains prevention and nub cleanup, but it does not establish a universal technique that restores already-stressed plastic to its original appearance.
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Gate on a broad, flat panel
Use a file or sanding stick to level the nub, then work through 400–600 and 1000+ grit. Keep the tool flat and check the panel from several angles. A broad surface gives you room to blend, but it also makes an over-sanded patch easier to see under reflected light.
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Gate beside a sharp edge
Leave a little more material during the first cut than you would on a broad panel. File toward the nub, not across the edge, and support the abrasive so it cannot roll over and round the corner. Finish with light strokes that stop at the edge rather than crossing it.
Gate near a panel line or recessed detail
Protect the line by limiting the abrasive area. A narrow sanding stick or a small folded piece of paper can localize the work, but the same staged progression still applies. If the nub touches the detail, remove material in very small increments and inspect after every few passes.
Gate on a curved or compound surface
Match the sanding support to the curve. Long, rigid strokes on a curved panel can create a flat patch, so use short passes and rotate the part frequently. The objective is a continuous contour, not merely a smooth spot at the former gate.
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You can prioritize structural fit and avoid unnecessary cosmetic work, but still remove enough material for adjacent parts to seat correctly. Check the assembly before committing to aggressive sanding; a nub that is invisible may still interfere with a joint or armor piece.
When a mark remains after careful cleanup
First, stop sanding and inspect the area under neutral light. If the surface is level but a pale tone remains, further sanding may change the part’s profile without guaranteeing that the color will return. That is the point to decide whether the location is visible enough to justify cosmetic work.
Leave the bare plastic alone
For a small mark on a hidden or low-visibility surface, preserving the original geometry is often the least risky choice. The official sources do not promise that any particular treatment will erase stress whitening.
Use an optional marker touch-up
Bandai’s finishing advice discusses markers as model-finishing tools, but the cited guidance does not claim that markers reverse stress in plastic. If you choose one, test the color and sheen on unused runner material first. Apply only after the nub is level and the surface is clean, because a marker changes appearance; it does not repair a distorted surface.
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If the part will be primed and painted as part of your normal build, a carefully leveled surface can be incorporated into that finish. Do not use paint as a reason to skip the staged cleanup: an uneven nub or altered edge can remain visible through the coating.
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Common mistakes and the corresponding fix
- Making one flush cut: The cut can stress the plastic and leave a white mark. Make a remote first cut and reserve the final removal for filing and sanding.
- Trying to remove the entire nub with nippers: This trades a little saved sanding time for less control at the part edge. Leave a nub and reduce it progressively.
- Starting with very fine abrasive: Fine paper is slow at leveling a prominent nub and can encourage excessive pressure. Use the 400–600 stage for leveling, then 1000+ for polishing, as in Bandai’s example.
- Using coarse abrasive too long: Sanding removes material and can flatten edges or erase detail. Inspect often and move to the finer stage once the nub is level.
- Using a hobby knife as the default: A knife can provide localized trimming but is not required. If you use one, make controlled cuts away from your fingers and leave enough material for sanding.
- Assuming a pale spot is only leftover nub: Stress whitening may extend into the part. Stop when the geometry is correct and treat any remaining cosmetic change separately.
- Skipping a test on unfamiliar plastic: The cited guidance does not compare clear, opaque, different colors or resin types. Test a finishing approach on spare runner material when practical.
- Holding the part over loose debris: A dropped nub can be difficult to identify among runner scraps. Clear the bench between cuts and keep small pieces in a tray.
- Working from one viewing angle: A mark can disappear head-on and show under reflected light. Rotate the part and inspect the edge, face and surrounding contour before declaring it finished.
A repeatable bench workflow
- Read the assembly step and identify the gate, the finished face and nearby detail.
- Place the nippers on the runner side of the gate.
- Make one separation cut that leaves a small nub.
- Set aside the runner and inspect the free part from multiple angles.
- File the nub down while preserving surrounding edges and lines.
- Sand with 400–600 grit until the filed transition is level.
- Polish with 1000-grit or finer abrasive using light pressure.
- Clean away sanding residue and check the surface under the light in which the model will normally be viewed.
- If a white area remains, decide between accepting it, applying a tested cosmetic touch-up, or incorporating the part into a planned painted finish. Do not keep removing plastic solely in the hope of reversing stress.
Troubleshooting by symptom
The nub is still visible after sanding: Confirm that the surface is actually high rather than merely discolored. If it is high, return briefly to the 400–600 stage; if it is level, do not chase the color by widening the sanded area.
The panel now has a flat spot: Stop using a rigid flat abrasive on the curve. Compare the contour with adjacent plastic and, if necessary, use a support that follows the curve with very light passes. The goal is to preserve the original profile, not to create a perfectly flat local patch.
A panel line is becoming shallow: Stop and inspect the line under magnification or strong light. Continue only with a narrower abrasive area and minimal pressure; a softened molded detail is a geometry change, not a stress-mark solution.
The marker looks too dark or glossy: Remove the cosmetic step from the decision until you can test another color or sheen on spare runner. Marker choice varies by color and finish, and it cannot correct an uneven surface.
The part no longer fits: Check whether the remaining nub is contacting a neighboring piece. For hidden surfaces, remove only the material that blocks assembly rather than sanding the entire area for appearance.
What the official guidance does—and does not—establish
Bandai’s material supports three firm practices: use plastic nippers to separate parts, avoid cutting too close to the finished piece, and clean the remaining nub through staged filing and sanding. The cited grit example is 400–600 followed by 1000+.
It does not provide a universal gate distance, a success rate, a guaranteed stress-mark repair, or comparative results for every plastic color, clear part, gate geometry or resin. Treat the sequence as a reliable starting method and adapt pressure, support and stopping points to the actual part in front of you.
For the manufacturer’s complete context, read Tips To Improve Your Gunpla, the beginner guide, the assembly tips and the tools catalog.
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Frequently Asked Questions
How far from the part should I make the first cut?
Bandai does not give a universal measurement. Cut on the runner side of the gate and leave a visible nub; the needed amount depends on the gate and the part’s geometry.
Can sanding always remove a white stress mark?
No. Sanding can level a nub, but the official guidance does not guarantee that existing stress whitening will return to the original color. Further sanding may alter the part’s shape.
Which grit should I use first?
After filing, Bandai’s example uses 400–600 grit for leveling and 1000-grit or finer abrasive for polishing.
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Do I need a hobby knife or model separator?
No. Plastic nippers, a file or sanding stick, and sandpaper are sufficient for the core method. A knife is optional, and Bandai’s catalog lists a separator as a tool category rather than a requirement for runner cutting.
Should I test on clear or unusual plastic?
Yes, when practical. The cited guidance does not compare clear, opaque, colored or resin materials, so test a finishing approach on unused runner before working on a visible part.
What if the gate is next to a panel line?
Leave the nub, localize the abrasive with a narrow stick or folded paper, and remove material in small increments. Inspect frequently so the recessed detail is not widened or softened.
Is a marker a repair for stress whitening?
A marker is only an optional cosmetic touch-up. Bandai discusses markers for finishing but does not claim they reverse plastic stress; test color and sheen on spare runner first.
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Are expensive nippers necessary?
The guidance supports plastic nippers but does not establish a price or performance ranking. Choose a cutter you can control and rely on staged filing and sanding rather than forcing a flush cut.
The Bottom Line
Cut the runner away from the part, leave a nub, and remove that nub gradually. File first, level with 400–600 grit, then polish with 1000 grit or finer while protecting edges and molded detail. If whitening remains after the surface is correctly shaped, stop removing plastic and choose acceptance, a tested cosmetic touch-up or your planned paint finish.
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