Maya Ellison
Filling seams in plastic models can be a challenging task, especially when dealing with older vintage kits or short-run kits, which often have large gaps in the joints. The process requires careful use of various tools and materials, such as adhesives, putties, and sandpaper, to achieve a smooth and seamless finish. While filling seams, it is important to pay attention to the specific techniques and precautions to ensure a successful and authentic-looking model. In this discussion, we will explore different methods and best practices for filling seams in plastic models, addressing common challenges and providing step-by-step guidance to help modellers create high-quality, seamless creations.
Techniques for filling seams in plastic models
| Characteristics | Values |
|---|---|
| Common problem areas | Wing roots and mating lines between both sides of the fuselage |
| Materials used | Adhesive, putty, sprue, lacquer thinner, sandpaper |
| Tools used | Dental probe, razor or X-acto knife, Q-tip, paintbrush |
| Techniques | Stretching sprue to fit the gap, using a surfacing agent, using a lacquer thinner to remove excess surfacing agent, sanding with various grades of sandpaper |
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What You'll Learn
Filling large gaps in plastic model joints
Gauge the Gap
First, it's important to assess the size of the gap. If it's approximately 1/16" or larger, you'll need to use a different technique than simply applying adhesive, as it won't be sufficient to bridge the joint. You may need to use multiple sizes of stretched sprue to fill the gap, as the size might vary along the seam.
Prepare the Sprue
The sprue is a thin piece of plastic that you'll use to fill the gap. Heat a plastic rod over a candle flame until it becomes pliable. Then, slowly pull the two ends apart to stretch it out and create a thin strip of plastic. You may need multiple sprues with different diameters to accommodate any changes in the gap size along the seam.
Apply Adhesive
Carefully lay the sprue along the seam, leaving a little extra on one side to give you something to hold onto. Apply liquid polystyrene adhesive along the sprue. Capillary action will draw the adhesive into the gap between the sprue and the seam, helping to create a strong bond. Be careful not to apply too much adhesive to avoid pooling along the joint.
Seal the Seam
Use a flat, narrow tool, such as a dental probe, to gently push the sprue into the seam. The adhesive will slightly melt the sprue, allowing you to squeeze it into the gap and create a tight seal. Use a sharp razor or X-acto knife to trim off any excess sprue material once it's in place.
Smooth the Surface
Even with the sprue in place, you may still see some contours or unevenness along the seam. To create a smooth, uniform surface, apply a surfacing agent, such as Mister Surfacer 500 from Gunze Sangyo or a similar product from Tamiya. These agents will help fill in any remaining gaps and create a seamless finish.
Sanding and Finishing
Once the surfacing agent is dry, it's time to sand the area. Start with rough-grit sandpaper (600 or 400 ALO) and work your way down to finer grits (320SIL and 150SIL). Sand the area until the surface feels smooth and even. If you lose any details during the sanding process, such as panel lines or rivets, use a scribing tool and a pin vise to restore them. Finally, apply a primer to your model, and it will be ready for painting!
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Using adhesive to fill seams
When using adhesive to fill seams in plastic models, there are several steps to follow. Firstly, gauge the size of the gap, as this will determine the amount of adhesive needed and whether additional techniques are required. If the gap is larger than 1/16", adhesive alone may not be sufficient to bridge the joint, and other methods should be considered in conjunction with adhesive.
The next step is to carefully apply a liquid polystyrene adhesive along the seam. It is important to not apply too much adhesive to avoid pooling along the joint. Use a flat, narrow tool, such as a dental probe, to gently push the adhesive into the seam. The adhesive will slightly melt the plastic, allowing you to squeeze it into the seam and seal the gap.
After applying the adhesive, use a sharp razor or X-acto knife to remove any excess material. Even with the adhesive in place, the contours of the seam may still be visible, especially for larger gaps. To address this, apply a surfacing agent to smoothen and even out the seam. Mister Surfacer 500 from Gunze Sangyo and Tamiya's grey surface treatment agent are recommended options.
Once the adhesive is dry, you can begin the sanding process to further refine the seam. Start with rough-grit sandpaper (600 or 400 ALO) and sand over the seam and any other areas that require smoothing. Repeat this step with finer-grit sandpaper (320SIL) to make the plastic surface smoother. Finally, use the finest-grit sandpaper (150SIL) to finish off and ensure a smooth and even surface. If you accidentally lose details during sanding, such as panel lines or rivets, you can use a scribing tool and a pin vise to restore them.
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Sanding process for a smooth finish
Filling seams in plastic models requires a careful sanding process to achieve a smooth finish. Here is a step-by-step guide on the sanding process:
Start with a rough sanding pass to create a uniform surface. Use sandpaper with a grit range of 400 to 600 for this initial step. This will help remove any excess material and create a consistent texture along the seam. It is important to feel for a rough surface during this step.
Progress to a finer grit sandpaper for the next step. Switch to sandpaper with a grit of around 320. Repeat the sanding process along the seam and other areas as needed. This step will help refine the surface and reduce the roughness created by the previous coarse grit.
For the final sanding pass, use the finest grit sandpaper. Opt for sandpaper with a grit of 150 or finer. Sand the same areas once more to achieve a smooth and refined finish. At this stage, the surface should feel noticeably smoother and more uniform.
If you have lost any fine details during the sanding process, such as panel lines or rivets, use a scribing tool or a pin vise to carefully restore them. This step ensures that the model retains its intricate details.
After sanding, the model is ready for further finishing processes. You can apply a primer to the sanded areas to prepare the surface for painting and your desired colour scheme.
Additionally, some modellers choose to use a lacquer thinner with a Q-tip to remove any remaining excess surface agent after sanding, creating an even smoother and more uniform seam. However, this step should be done carefully to avoid melting the plastic.
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Welding plastic seams
To begin, ensure your plastic surface is completely clean. Use soap and warm water to scrub off any debris, and if necessary, wash the plastic with mild dish soap or detergent to eliminate dirt, grease, or other debris. For stubborn stains, try using a liquid solvent called methyl ethyl ketone (MEK). Once clean, dry the plastic off with a clean, lint-free cloth.
Next, sand down any jagged edges to create a smooth surface. If you are joining two pieces of plastic, ensure they fit together properly. Warm up your welding tool—this could be an electric welding gun, an iron, or a simple open candle flame—and run it along the seam until the plastic becomes soft and pliable. Adjust the pieces as needed so they fit together seamlessly.
If desired, add thin plastic strips along the seam to strengthen the weld. You can also use a welding rod that matches the type of plastic you are working with. Melt the end of the rod and attach it to a clean spot on the plastic, filling in any gaps.
Finally, finish the weld by smoothing it out. You can use a flat, narrow tool, such as a dental probe, to gently push the plastic into place and create a uniform seam. If needed, sand the welded area again using progressively finer-grit sandpaper until the surface feels smooth and fine.
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Filling materials for gaps and seams
Filling gaps and seams in plastic models is a common challenge, especially when dealing with older vintage kits or short-run kits. While some gaps can be addressed with adhesive, larger gaps and seams require more creative solutions. Here are some materials and techniques to fill and smooth these imperfections:
Epoxy Putty
Epoxy putties, such as "Green Stuff" or "White Stuff" by Squadron, are versatile gap-filling materials. They can be sanded smooth and, when used in well-ventilated areas, can be chemically smoothed as well. However, they tend to shrink over time, making them more suitable for small gaps.
Water-Based Pastes
Water-based pastes, such as those offered by Vallejo or Perfect Plastic Putty, are easy to apply and can be smoothed with a wet finger or cotton bud before drying. While they do shrink slightly, they are ideal for filling small gaps in plastic models.
Mr Surfacer 500
Mr Surfacer 500 is a thick primer that can be thinned with a mixture of lacquer thinner and IPA for use as a primer. It can also be used as a gap-filling material but requires sanding to achieve a smooth finish.
Sprue and Adhesive
For larger gaps, you can create a filler by cutting and stretching sprue to fit the gap. Sprue is the plastic material that connects the individual parts of a model kit to the sprue tree. By heating a piece of plastic rod over a candle flame, you can slowly stretch it to the desired diameter and length. Apply liquid polystyrene adhesive along the sprue, and use a flat, narrow tool to gently push it into the seam. Cut off any excess sprue, and apply a surfacing agent to smooth and even out the seam.
Model Putty and Welding
Model putty is an effective filling material for gaps, pits, and scratches. It can be applied with a toothpick, but care must be taken to avoid smudging or applying it to unnecessary areas, as excess putty can hide surface details. For plastic model aircraft, Tenax-7R can be used to weld the left and right sides of the fuselage together. By applying medium pressure and gradually increasing it over 10 seconds, you can create a firm bond.
When filling gaps and seams in plastic models, it's important to have a variety of tools and materials at your disposal. Sanding, priming, and painting techniques are also crucial steps to achieve a smooth and authentic finish.
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Frequently asked questions
Epoxy putties are recommended for filling gaps in plastic models, but other materials can be used, such as green stuff, white stuff, water-based pastes, Mr Surfacer 500, or a mix of lacquer thinner and IPA.
First, use a paintbrush to apply a fast-dry formula like Tenax-7R to weld the plastic parts together. Then, use model putty to fill in any remaining gaps, scratches, or pits. Once dry, sand the model using various grades of sandpaper, from rough to fine, until the surface is smooth. Finally, apply a primer to the model before painting.
For large gaps, you can use a technique called "stretching sprue." Heat a piece of plastic rod over an open flame and slowly pull the two ends apart until you have a diameter that fits the gap. Then, apply liquid polystyrene adhesive along the sprue and use a flat, narrow tool to push it into the seam. Cut off any excess sprue with a razor or knife.
