Welcome to the first resin kit tutorial! Although the kit used in this tutorial is slight departure from our usual space gun/ship kit, this kit has a TON of parts, three different types of resin, and covers a lot of techniques that should help you through other kit builds and more if you have never touched a resin kit. So, let's get right to it. This tutorial will serve as a general resin kit tutorial, as well as a specific tutorial for the 1/6 scale Star Wars AT-RT Walker resin kit we produce. Although I don't recommend this specific kit as a first kit, it is certainly do-able for the motivated maker.
We'll start off with what you'll need for build and prep. At a bare minimum, you'll need 220 and 400 grit sandpaper, a hobby ("Exacto") knife, cyanoacrylate ("Super Glue"), and on this kit specifically, some way to drill a 1/16th inch hole. Things like scissors, a model saw ("exacto saw") and a rotary tool ("Dremel") are highly recommended in general for kit building.
Trimming Raw Parts
This is where some good old fashioned scissors can help. We usually remove most of the flashing from our parts, but in cased we missed some, you want a closer cut, or got a kit from a 3rd party with some heavy flashing, this step can save time. Simply do your thing and cut the flashing off. Be careful not to cut too close or at an angle.
Sanding Open-Faced Parts
The easiest parts to prep are cast from "open faced molds". These are parts that are relatively simple in geometry and have been made in a such a way that one surface doesn't show, has no detail and can be sanded flat, or is simply glued to the face of another part. That means no seam line, and usually one simple surface to take care of. For this, we use 220 grit sandpaper and a flat surface (a machined, flat surface is ideal. However, at this small a scale a piece of wood etc. will do fine)
Simply take the part, and sand it back and forth on the 220. Try and apply pressure evenly with your fingers, or the part may come out a little crooked. Slow, precise, and firm sanding is ideal as opposed to quick, vigorous sanding. we don't want to create a rounded surface on the bottom.
Check your work as you go. This particular part has a fairly simple thickness/ geometry and is easy to see if one side is higher or lower. Don't always follow the line/ridge at the edge of the face you're sanding, it can be misleading if the mold was trimmed after production. I like to clean up that small edge with a light pass of 400, just to round it out a tad. Can you tell the right side is a tad thicker, even after sanding? It won't matter for this application, but is something to be aware of.
Some open faced parts are made to be glued together. This is designed to reduce production time/costs, or simply to orient the casting process in a way to minimize or eliminate bubbles. This exhaust muffler assembly needs to be surfaced and glued. You can also see the original state of the back of the part we just sanded.
You know what to do! (Hopefully)
Gluing Open Faced Parts and Assemblies
Now the scary part. Glue. If you are using "super glue", you only get a +/-3 shot at this. on surfaces that have been sanded with 220, the bond is instant. You only need 2-3 tiny drops on something this small. The bond is incredibly strong. If you wish, you can also mix up some 5 minute epoxy instead. It is much more forgiving in the sense that you have way more time to position your parts before they set. On the downside, they often need to be clamped, and on parts this small, it is near impossible, and the parts can "float" around a bit before they settle. Epoxy is also thick and oozes from cracks. I also like to scrape the edges with an exacto knife before I glue them down to ensure no "lip" remains. More on exacto-based cleanup in a bit.
On larger, or multi-part assemblies, injecting super glue (Not the gel type super glue, which is awful for everything) into the cracks and crevices after holding the parts in their ideal position can work, as long as the glue has room to seep into the area. This doesn't work with perfectly matched up parts, so you'll need to intentionally leave a gap or simply use it on an application where fitment isn't near perfect. This AT-RT assembly is a good example, as finding alignment here can be tricky, and once the glue is laid down and the parts touch, that is it. This method works backwards from that, and while can result in some runny glue marks, will ensure your alignment is exactly where you left it. Just be sure to sand the mating surfaces with 220 grit.
Cleaning up Bubbles
We're going to go back to that rectangular part to cover the bane of resin kit building: Dealing with bubbles. There are two types of bubbles. Positive, and negative. Positive bubbles are a result of bubbles from the silicone process and therefore all subsequent casts will have these. Negative bubbles are from the casting process and vary part to part. Positive bubbles, as the name implies, are typically easy to deal with and can usually be removed by simply scraping them away with a hobby/exacto knife of the like. Be careful if you are using a brand new blade, the tip can easily break off and hit you in the eye. Here's an example of some pesky little bumps we have to deal with. At least there's medicine for these...
As you can see below, simply dragging the exacto parallel to the surfaces in question made quick work of the positive bubbles. We're going to remove all of them in this inside area, as well as on the top surface inside those two parallel grooves.
Sadly, we aren't done yet. This part has two, tiny negative bubbles we should probably address. Counter to what you might think, we're actually going to make those holes larger by digging them out slightly with an exacto knife. This will make them easier to fill.
Now we fill it. This is a point of contention for makers. Do we use body filler (Bondo)? Spot putty? Apoxie sculpt? Super glue and baking soda? That all depends on the scale of the imperfection, as well as which method you prefer. Personally, we loathe body filler (Bondo) on anything that isn't a large, smooth surface. For most things, especially things this small, we prefer the super glue method as it is by far the fastest and the materials are already in front of us. We start by sanding the surrounding area of the hole with 400 grit until some resin dust fills the hole. (or you can use some from your sanding excursion earlier, or use baking soda) Then, we add a tiny drop of super glue.
Here, we can wait for the glue to dry completely before sanding. Personally, we like to remove a little of the excess, and when it is about half way cured, start sanding it with 220 or so. This will immediate pack the hole with dust and cure the glue, at the expense of gunking up your sandpaper, so we use scraps for this. It should look something like this when done. You can repeat the process as many times as needed to fill the hole, and on the plus side, since it uses the same filler that the part is made from, it is very close in color. This is good for kits that encourage the builder to use the resin color of the kit as the base color, such as our Arwing kit and black M96 Mattock kit.
For general seam lines and larger bubbles, we like to use a combination of the above super glue/fill method, as well as some careful exacto handling. Here's a large bubbled area on the front shroud of this kit that required about three passes of the super glue method and some 220:
Hopefully, whatever part you are working on has some shallow seam lines and can be partially (If not mostly) addressed with an exacto blade simply by scraping it. Use as new a blade as possible for this, and ONLY scrape away from you. Do not scrape back and forth, as it causes vibrations and a wavy finish, and more importantly can lead the user cutting themselves. This method is extremely quick and the results are hard to argue with. Provided the kit is of highest quality, and not from an older mold) Pictured here is a partial seam line on the exhaust muffler we glued together earlier. Any imperfections leftover after scraping should be treated with the super glue method. (Or your filler method of choice)
Transparent Resin parts
Resin parts can sometimes be printed vs cast. For small parts like this, printing in clear resin is much faster and more economical than casting resin. However, prepping the part is the same. Since clear parts are typically for illumination, we need to diffuse the light. We can do this many ways, but the most simple step is to sand all of the surfaces of the part with 400 grit sandpaper. This is a tiny leadlight lens, and luckily it is flat and quick to sand.
You can fold a small piece of sandpaper in half one or two times to create a firm, thin "file" of sorts. This helps get in nooks and crannies without the need for specialized files. However, the thin edge will wear down fast, so you'll have to periodically re-fold the sandpaper and use a different location on the sandpaper. In this case, we need to kit this tiny crack with 400 grit.
In cases like this where a clear part needs to be installed it will need to be glued after painting, but for this kit/example we are going to glue it in early. If you are lucky, the part will simply press into place firmly. Ours didn't though, so a few TINY drops of super glue were used. They make special glues for this sort of thing (Model aircraft canopy glue) if you want to try that, but because we have access to the front and back of the lens the excess glue can simply be sanded off with more 400 grit. Super glue is typically runny/watery and gets into little cracks to help bonding, but on the downside can leak into places you don't want. One of my drops leaked into the back here, but sanding made short work of it.
Sprue Clean up/sawing
Our kits ship out with the sprues already cut, but in case we missed a bit, or you got one from another vendor, there are two options. The Dremel/rotary tool is probably the most common method out there and for good reason. You can do most operations on one tool and have tons of options for bits. A lesser used method is the exacto saw. This is a far less dusty, and often more precise for the bigger. One slip up with the dremel and you can create a lot more work for yourself. The exacto saw is a slower and safertool for this sort of thing. Pictured here is the removal of two casting vents from a resin part.
Lastly, and possibly most importantly, give all of your parts a sand with 400 grit sandpaper, ideally a wetsand. (Wetsanding, as the name implies is sanding while the surface of the part is wet. Some sandpaper adhesive will deteriorate in water though, so try and get sandpaper made for wetsanding if you plan to do it a lot) The reason for this is during the casting process, any maker worth their salt sprays the mold with a release agent which by design makes it hard for things to stick together. The molds are then brushed with powder to soak the resin into the nooks and crannies, resulting in less bubbles. The downside is the powder and resin adsorbs most of the mold release and gets embedded into the surface of the part. This means that paint won't stick well to a resin surface that hasn't been prepped. Sometimes it seems like it does, and then chips off later. Be VERY sure to prep any/all surfaces before paint! Below is a picture of 3 different resin materials, all wet-sanded with 400 and ready for paint.
Part 2 soon!