Seam Line Removal – Gunpla Builders World Cup (GBWC) Prep Series Part 1

Ever stepped back from your Gunpla and noticed a glaring seam line ruining the look? With the Gunpla Builders World Cup (GBWC) submission deadline coming up, it's not too late to improve your build. Over this 5-part series, I’ll be sharing techniques to help elevate your Gunpla, making it stand out from the crowd. 

Nothing breaks immersion on a finished Gunpla model like a glaring seam line down the middle of a weapon, shoulder or leg. If you have ever stepped back from your build and felt something was off, the seam lines could be the culprit.

Seam line removal is one of the biggest leaps you can make in improving build quality. From prepping for a full custom painted model or looking to level up your out-of-box builds, understanding and applying seam removal techniques is essential.

In this post, I'll break down the different methods for seam line removal/hiding, the tools that you’ll need and when I use each technique for the best results. Let's get into it!

What are Seam Lines

Seam lines are the resulting gaps or ridges where two parts meet, caused by the mould division during kit manufacturing. They can reduce realism and distract from the overall aesthetic.

Most Bandai Gunpla kits include seam lines, but their prominence varies. Typically, High Grade (HG) and Super Deformed (SD) kits feature more noticeable seams, while Master Grade (MG) and Perfect Grade (PG) kits often have fewer or less visible ones. 

Removing these seam lines is a key skill for competition-level and showcase builds. Left uncorrected, they can highlight imperfections and diminish the realism of your model.

Tools You’ll Need

Let's get into the tools you will need for these processes. The basic tools I use for this process are some nippers (Godhand nippers ) for removing the parts of the runner, plastic cement (Tamiya or Mr Hobby are my go-tos), sandpaper or sanding sticks  (400 - 1000 grit), and my fingers or rubber bands to apply pressure on the parts.

Some optional tools you could use are a hobby knife, putty/filler or masking tape, depending on your budget and availability. These tools are readily available and affordable from hobby stores or online and are linked for ease.

Safety note: Work in a well-ventilated area when using cement and putty. Use proper protective gear as needed. 

The Cement Method 

Now that we know the tools, let's get into the techniques. I always begin with a review of the seam line on the kit and the impact on movement. Most seam lines will be on the upper and lower arms, upper and lower legs, weapons and inner frames.

Once this is done, it helps me determine the method to use. The cement method is good for easy access parts with minimal impact on movement. For other seam lines, there may be a need for other techniques with putty or scribing

When ready, apply a generous amount of plastic cement along the seam line while the parts are separated slightly. The goal is to have enough of a reaction to melt the parts together when pressure is applied.

Next, I'll push the parts together firmly and ensure that some melted plastic is visible. At this point, I want to give a warning about the pressure. The cement makes the pieces malleable, and with this comes an increased chance of minor cracking if the pressure is too much. Try to maintain firm, even pressure on the parts going together.

If you aren't getting enough plastic squeeze out, you can apply extra cement onto the seam line and then more pressure to get a squeeze out. The parts then need time to cure in the new position. Here is where the rubber bands or masking tape may come in handy.

When you relax the pressure on the parts, the melted plastic may relax and come away as well. If this happens, you need to maintain the pressure on the parts while it cures to prevent gaps or pin holes from forming in the curing process (usually 2-4 hours, depending on the cement).

Once cured, the excess can be scraped with a hobby knife or sanded. My preference is sanding the excess, but scraping with a hobby knife is just as good. I use sanding sticks for flat surfaces to ensure the transition is smooth. I use sandpaper for curved surfaces to avoid creating flat spots and maintain the rounded transitions.

You can practice the above technique on any spare parts you may have to help build confidence. The key is to take your time when starting this process and not to rush the curing process.

When and How to Use Putty/Filler

Putty is my ultimate corrector once the cement method is completed. I have found that putty can be very useful for gaps in weapons and armour parts.

There are different types of filler and putty, including water-based, lacquer based and 2 part epoxy. It pays to keep in mind that if you are looking to use this method, there will be a need to paint your kits, as most putties are different colours and need painting to blend the pieces together

You can use any type that you can access, and there will be a learning curve based on the type you choose. Water-based putties are my go-to for ease of application, but I have also used epoxy putties for larger modifications.

The technique involves applying a thin layer of putty over the seam line and letting it dry fully. Same as the cement method, sanding the pieces to achieve the transition is necessary. The main difference is that once you have sanded the pieces, you’ll need to prime the pieces to check for impurities (pin holes or low points).

Final Checks - Priming and Surface Preparation

I recommend priming after the seam work is done. For the cement method, you can sometimes polish the surfaces enough to mask the transition if the method is applied well.

I will generally prime the parts in an opposite colour, grey primer on white parts, white primer on black parts, etc, to highlight imperfections that can be missed. It may seem odd, but using a single colour on a part that you are working on is enough to get a different look and really assists in seeing the imperfections.

If there are imperfections, you can apply putty into holes or depressions or re-sand bumps and imperfections further and re-prime to check the error. This step will also ensure that any paint work you do goes on smoothly and hides the seam lines you have worked so hard to remove!

Removing seam lines might feel intimidating at first, but it’s one of the most rewarding upgrades to your gunpla building skill set. With a bit of practice and patience, you can get your kits looking cleaner, more professional and ready for the spotlight at GBWC. 

For more tips and tricks, follow me on socials @rosie_custom_gunpla