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To separate a Gundam part cleanly, do not cut the gate flush with the finished surface in one move. Use plastic model nippers to cut the runner a short distance from the part, leave a small nub, and remove that nub in stages with a file, 400–600 grit abrasive, and 1000+ grit polishing paper. Bandai warns that cutting too close can create white stress marks in the plastic; gradual cleanup reduces the chance of damaging edges or molded detail.
What causes a white stress mark?
A kit part is connected to its runner by a small bridge of plastic called a gate. After the part is cut free, the gate leaves a nub that must be removed. If the nipper jaws are placed directly against the finished part and the gate is severed in one forceful cut, the surrounding plastic can be stressed. Bandai describes the result this way: “When you cut too close to the required piece, you could leave white marks which are caused by stress from the plastic.” You can read the complete guidance in Bandai’s Tips To Improve Your Gunpla.
The white area is therefore not always just leftover nub material. A nub can usually be reduced mechanically, but a stress mark that remains after careful cleanup is not guaranteed to return to the original color. Sanding also removes the surrounding surface, so aggressive sanding can flatten a panel edge, round a corner, or erase nearby molded detail.
The basic method at a glance
- Identify the gate and the part’s visible surfaces.
- Make the first cut in the runner, leaving a small amount of gate attached to the part.
- Make a controlled second cut or file the remaining nub down, rather than forcing a flush cut.
- Sand the area with 400–600 grit abrasive.
- Refine or polish with 1000-grit or finer abrasive.
- Inspect the surface before deciding whether any cosmetic touch-up is necessary.
The grit sequence is Bandai’s example, not a universal recipe for every plastic color, gate shape, or finish. Adjust the amount of work to the part: a broad flat surface tolerates more controlled sanding than a sharp armor edge or a tiny raised detail.
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Tools you actually need
| Tool | Role | How to use it safely and effectively |
|---|---|---|
| Plastic model nippers (side cutters) | Separate the part from the runner | Use them for the initial cut with a small margin of gate left for cleanup. Bandai’s beginner guidance lists plastic nippers for cutting parts from sprues. |
| File or sanding stick | Reduce the remaining nub | Work slowly and keep the abrasive localized. A file removes material quickly; a sanding stick gives a broader, more controllable contact area. |
| 400–600 grit sandpaper | Level the filed area | Use light strokes until the nub blends into the surrounding surface. Bandai gives this range as an example. |
| 1000+ grit sandpaper or polishing abrasive | Refine the surface | Use it after the coarser stage to reduce visible sanding texture. Bandai recommends 1000 grit or higher for polishing. |
| Hobby knife | Optional localized trimming | Useful for a small remaining ridge when you can control the blade, but it is not required for the core nipper, file, and sanding process. Cut away from fingers and protect the part from slips. |
| Marker | Optional cosmetic color touch-up | Color and sheen may not match the surrounding plastic. A marker can disguise a blemish in some finishes, but Bandai’s cited guidance does not establish that it reverses stress whitening. |
Bandai’s official tool catalog lists model sanding sticks and a model separator. The separator is a separate tool category; it is not required to cut parts from runners when you already have plastic nippers.
Step-by-step: remove a part without creating a stress mark
1. Find every gate before cutting
Look around the part and identify each point where runner plastic meets the part. Some pieces have one obvious gate; others have several, including gates on edges that will be visible after assembly. Check the kit manual so you know which surfaces will remain exposed. Planning the cut direction prevents you from putting nipper pressure on a thin fin, a sharp corner, or a small molded detail.
Separate the visible “finished” surface from the runner side in your mind. The goal is to keep the initial force away from the surface that will be seen. If a gate is attached to a delicate edge, leave more material than you would on a broad, flat area and plan to file it down.
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Place the nippers on the runner, not against the part. Leave a short length of gate attached to the part and cut the runner free. This is the critical prevention step: the runner section absorbs the cutting force instead of transferring all of it into the finished surface.
Do not treat a flush first cut as a time-saving shortcut. Bandai specifically cautions that cutting too close can leave stress-related white marks. The exact safe distance depends on the gate and the part, and the official guidance does not set a universal measurement. When in doubt, leave a little more material for the next stage.
3. Free the part from the remaining runner
After the first cut, reposition the piece so the remaining gate is easy to reach. You can make a controlled second nipper cut farther from the part, or move directly to a file or sanding stick. The purpose is to reduce the nub without levering or twisting the part.
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Use the nipper jaws only where they have a clear path. Squeezing across a thin fin or a raised detail can crush the plastic even if the gate itself is small. If the gate is awkwardly oriented, remove more of the surrounding runner first and then approach the nub from a better angle.
4. File the nub down gradually
Hold the part securely without covering the area you are working on. File the nub in short, controlled passes, checking the profile frequently. Keep the file on the nub rather than sweeping broadly across adjacent armor. Stop before you reach the surrounding surface; the remaining discoloration or tiny ridge can be handled with finer abrasive.
Bandai’s assembly advice also emphasizes cutting parts from the runner and sanding rough edges. Its instructions are available on the Assemble page. The practical implication is to treat cleanup as a separate operation instead of trying to remove all material with the nippers.
5. Sand with 400–600 grit
Use 400–600 grit paper or a sanding stick to level the last part of the nub. Sand in a direction that keeps the abrasive on the gate area. On a flat panel, a small rigid sanding surface helps preserve the plane; on a curved part, use an abrasive that follows the contour without digging into one spot.
Make a few strokes, brush away the dust, and inspect. Continuing until the whole surrounding panel has been abraded is unnecessary and can change the surface profile. The objective is a level transition, not removal of an arbitrary amount of plastic.
6. Refine with 1000 grit or finer
Once the nub is level, switch to 1000-grit or finer abrasive to reduce the coarser sanding texture. Use lighter pressure than in the previous stage and keep the work area small. This step is especially useful when the part will remain bare plastic and the cleaned area must blend with the factory surface.
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Bandai presents 400–600 grit followed by 1000+ grit as a general example. It does not claim that these exact grits produce an identical result on every color, gate location, or finish, so inspect the part rather than following the numbers mechanically.
7. Inspect under direct light
Rotate the part under a bright light and view it from several angles. Look for three separate conditions:
- Remaining nub: raised plastic is still present and needs more localized filing or sanding.
- Sanding texture: the area is level but looks rough compared with the surrounding surface; continue with finer abrasive.
- Stress whitening: the surface is level but remains lighter than the surrounding plastic; more sanding may remove material without restoring the original color.
Separating these conditions prevents you from over-sanding a mark that is no longer a height difference.
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Gate on a broad flat panel
Leave a modest nub on the first cut, then use a flat file or sanding stick. Keep the abrasive face parallel to the panel so you do not create a shallow hollow around the gate. Finish with the 400–600 and 1000+ grit stages, checking that the panel remains flat.
Gate on a sharp corner
Protect the corner by removing most of the runner before bringing in the file. Use short strokes that stop at the edge rather than crossing over it. A hobby knife can be used for a tiny ridge only if you can control the blade; it should be an optional trimming aid, not a substitute for the stress-avoiding first cut.
Gate beside a raised detail or panel line
Work with the abrasive confined to the nub. Broad strokes can soften a line or reduce the height of a nearby detail. If the gate touches a narrow recess, remove material in several small passes and inspect after each pass instead of trying to make one deep cut.
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Gate on a curved surface
Match the abrasive to the curve and use light pressure. A rigid, flat tool can create a flat spot on a rounded surface. The official guidance does not specify a special method for curves, so rely on frequent visual checks and stop as soon as the transition is smooth.
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What to do when a white mark remains
First confirm that the area is actually level. If a raised nub remains, continue the normal file-and-sand sequence. If the surface is level and the lighter color persists, treat it as a stress-related discoloration rather than assuming it is removable excess plastic.
There is no universal repair established by the cited Bandai instructions for restoring an already-stressed area to its original appearance. Further sanding may reduce the visible boundary, but it also removes surrounding plastic and can distort the part. Stop when the risk of changing the shape is greater than the cosmetic benefit.
An optional cosmetic touch-up can be considered when the mark is visible after assembly. A marker may alter the color or sheen, and a mismatch can be more noticeable than the original mark. Bandai’s finishing advice discusses markers as modeling tools, but it does not promise that they erase stress whitening. Test any color on unused runner material first when practical, especially if the finish of the completed part matters.
Common mistakes and their fixes
| Symptom | Likely mistake | Correction |
|---|---|---|
| White crescent appears immediately after cutting | The nipper cut was made too close to the part or under excessive force. | Do not chase the mark with a deep cut. Level only the nub, then decide whether a conservative cosmetic touch-up is preferable to more sanding. |
| A visible bump remains after sanding | The abrasive did not reach the highest part of the nub. | Return briefly to a file or 400–600 grit, keeping the work localized, then refine with 1000+ grit. |
| The panel looks hollow or rounded | Sanding spread beyond the gate or pressure was concentrated in one spot. | Stop removing material. On future parts, use a smaller sanding area, lighter pressure, and more frequent inspections. |
| A panel line or raised detail has softened | A file or sanding stick crossed the detail repeatedly. | Use shorter strokes that terminate before the detail. The lost geometry may not be recoverable by sanding alone. |
| The mark looks smooth but remains lighter | The remaining issue is likely discoloration rather than a raised nub. | Stop aggressive abrasion and evaluate an optional color touch-up instead. |
| The part cracks or bends during separation | The runner was twisted, leveraged, or cut while supporting a thin feature. | Remove more runner first, change the cutting angle, and support the part without squeezing delicate details. |
Clear, unusual, or nonstandard plastic
The cited official guidance gives a general process but does not compare outcomes by opaque color, clear plastic, or resin type. Do not assume that a grit sequence proven acceptable on one material will look identical on another. Check the kit manual and, when there is spare runner material, practice the cut and finish on that unused section before working on a visible part.
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For a clear piece, a cloudy or scratched area can be especially conspicuous. Use the same stress-avoidance principle—leave material, then reduce it gradually—but treat polishing and final appearance as material-specific questions rather than guaranteed results from the general Gunpla instructions.
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A repeatable workflow for a complete kit
- Prepare the workspace. Have nippers, a file or sanding stick, 400–600 grit abrasive, and 1000+ grit abrasive within reach. Keep the kit manual available for part orientation.
- Group parts by task. Work on a small batch so you can maintain the same cut-and-clean sequence without rushing.
- Inspect before each cut. Locate gates and identify which edges will be visible after assembly.
- Separate in two stages. Cut the runner away from the part first, leaving a nub, then reduce the nub with controlled tools.
- Use progressive abrasion. File or use a sanding stick, move to 400–600 grit, and finish with 1000+ grit when the surface requires refinement.
- Check the profile and color separately. A level but lighter area is not the same problem as a raised nub.
- Reserve touch-up for the end. Only after the plastic is clean and level should you decide whether a marker or another finish is worth the color and sheen trade-off.
Why the two-stage cut is worth the extra time
A flush cut appears faster, but it concentrates the separation force at the finished surface and leaves less material for controlled cleanup. The two-stage approach assigns different jobs to different tools: nippers remove the runner, while files and abrasives remove the nub. That division gives you more control over how much plastic is removed and where it is removed.
It also makes errors easier to stop. A slightly oversized nub can be reduced in several inspections; a stress mark or gouged edge created by an overly close cut may remain visible or require a more involved finish. The process does not guarantee a mark-free result on every gate, but it follows Bandai’s specific warning and its staged cleanup example.
Official guidance
- Bandai, “Tips To Improve Your Gunpla” — explains stress-related white marks, nub filing, 400–600 grit sanding, and 1000+ grit polishing.
- Bandai Hobby Site Satellite, “Customer Support – BANDAI HOBBY SITE” — lists plastic nippers and beginner sprue-separation guidance.
- Bandai Hobby Site Satellite, “Assemble” — covers cutting parts from the runner and sanding rough edges.
- Bandai Hobby Site Satellite, “Tools” — lists model sanding sticks and a model separator.
Frequently Asked Questions
Can I cut the gate flush with the part to avoid sanding?
No. Bandai warns that cutting too close can create white stress marks. Leave a small nub and remove it with filing and progressive sanding.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesWhat grit should I use on a Gundam nub?
Bandai’s example uses 400–600 grit to sand the area and 1000 grit or finer to polish it. Treat those values as general guidance and stop before nearby detail or edges change shape.
Will sanding always remove a white stress mark?
No. A level area that remains lighter may be stress discoloration rather than excess nub. The cited official guidance does not establish a universal repair that restores the original appearance.
Is a hobby knife required?
No. Nippers, a file or sanding stick, and sandpaper cover the core method. A hobby knife is an optional localized trimming tool that requires careful blade handling.
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