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How to Build a Resin Kit

I am often asked, "How is building a resin kit different from a regular styrene kit?", and "Can you tell me how to build a resin kit?" It seems to be feared; a mystical and arcane process for most model railroaders. Well, it isn’t as easy as falling off a log, but if you are willing to try a few new things and invest in a few simple tools (some of which you may already have) then you can learn to build resin cast models, sometimes also known as ‘craftsman’ kits. And you’ll find they are very rewarding to build as well.

By way of explaining the difference between commercially made mass produced styrene kits and short-run resin craftsman kits, lets start by looking at how they are made. A styrene kit, usually offered by large model companies like Walthers or Athearn, is mass-produced in lots of tens of thousands. Engineers and machinists spend many hours to develop a highly-engineered model and cut the molds for it from steel or aluminum. It is a high-speed process where heated styrene plastic is injected into those molds under high pressure over and over again. All of the labor and machinery is expensive to pay for, but because of the speed of production and the volume of models produced the individual models can be sold at relatively low prices and the company still turns a profit.

A resin craftsman kit, on the other hand, usually comes from a garage based business with one or two people who do everything. They can't spend tens of thousands of dollars on sophisticated tooling and expensive injection-molding machinery. In many cases simple RTV rubber molds are made from hand-crafted parts, made by a modeler just like you. A two-part resin plastic is mixed and poured into the molds, which takes time to set up. Often only a few sets of parts can be made in a day, maybe 15 or 20 full kits on a good day. Production runs are far shorter, often running from 250-500 kits, often far less.

Of course the real difference is in the offerings of the two companies. The big professional company can make the same model available to nearly everyone who wants one, but it has to have wide appeal or the economics don't work. A resin kit producer serves the smaller niche market where the big boys can't go. They can offer the special, the unusual, the limited interest models the big guys can't afford to touch. They make it possible for you to get a wider variety of models you'd otherwise be unable to get but for kitbashing and scratch-building yourself. And even with all their expenses (resins and molding rubber aren't cheap), they manage to keep the resin kits price-competitive with the injection-molded styrene kits.

So now we know why the kits are different. Let’s talk about how to build a resin kit.

Resin kits are a little different from the styrene kits you are probably used to. Unlike styrene molding, where pellets are heated into liquid and forced into steel molds under high pressure, resin is usually a two-part liquid polyurethane plastic that is mixed and then poured into a mold. The liquid resin goes through a chemical reaction that links the molecules into long polymers, solidifying it. Where the styrene process which is completed in seconds, the resin process usually takes from 5-30 minutes, sometimes longer, per set of parts. As you might expect, the polyurethane (PU) resin castings have different properties than good ol' styrene.

For one thing, resins can't be bonded with styrene glues. Testors, Tenax 7R, even MEK or Acetone won't do a thing to PU resin. The preferred glues for working with PU resin are Cyanoacrylate Cement, also known as Superglue, CA, ACC, Krazy Glue and others; and Epoxy. (I will refer to cyanoacrylate as CA for the remainder of this article.) Both CA and epoxy have their weak and strong attributes, and often one or the other may be called for even in the same model. Epoxy is super strong, but it takes time for the bond to set up; anywhere from 5-30 minutes depending on the type you buy. This means clamping or taping parts together so they won't move while the glue sets up. CA on the other hand sets up in seconds, faster if you use a 'kicker' that sets it instantly. But CA doesn't have much shear strength (parts can pop off if stressed from the side), and can fail completely if exposed to cold conditions. I use both depending on the situation, and try not to leave my models in the trunk of the car during winter months. :)

You also need to be really careful about keeping your fingers away from the joint area while the glue is active or you'll quickly find the model glued to your fingers. Don't panic, acetone (nail polish remover) will free you quickly, but you may want to wear safety glasses when working with this stuff, as gluing your finger to your eyelid is much more delicate operation to handle. Not to mention the fumes are quite strong and irritating to the eyes at times – which is what leads to touching your eyelids when you shouldn’t.

(If you use CA to assemble your resin models you can take advantage of one interesting chemical feature - Acetone doesn't bother PU resin, but it is a solvent for CA. If you make a mistake you can brush acetone onto the joint and it will dissolve the CA glue, letting you take the joint apart and try again. Try that with styrene! You can also use acetone and a Q-tip to scrub the glue residue off the model after you get your finger unglued from it.)

So what's in the box?

Opening up your first resin kit may be a surprise if you haven't seen one before. What you will usually find is a bunch of flat parts that may have a lot of flash (paper-thin extra resin that squeezed out of the mold when the part was made), extra parts for details made from resin, styrene, pewter or brass, wire, a sheet or two of instructions, and possibly decals. Some kits may include one-piece body castings, which are much simpler to build and make life a lot easier. It's quite intimidating on first glance, but you can do this! It isn't as bad as it looks.

Your very first step should be to read through the instructions carefully and completely. I know most of us are guys and it's a pride thing, but swallow it and suck it up! It's often very important to do things in the prescribed steps or you may regret it after. There may also be suggestions of how to do something that may not be obvious and could save you time and heartache later. This is even more critical on models that have lousy instructions - unfortunately there are some like this out there. You'll want to familiarize yourself with all the parts and how they go on the model and when before you start anything.

After reading the instructions, collect all the parts that are not water-adverse and place them in a bowl with warm soapy water. Out of the box, almost all PU resin parts are coated with a slick substance called mold release that helps keep the resin from sticking to the molding rubber. You need to wash this off before proceeding or the glue may not bond the parts together, and painting will be impossible. In some rare cases the parts must also be washed in acetone, it should say in the instructions if this is needed. Clean the parts well (I use a soft toothbrush), rinse in clean water and blot dry with a paper towel.

The next step is to clean up the flash on the parts. GO SLOWLY AND TAKE YOUR TIME WITH THIS STEP. I cannot stress this strongly enough. Resin models are made or broken here. Use a combination of tools - hobby knife, sandpaper, files. You will also want to keep your machinist square handy to check the squareness of corners or the straightness of parts. A steel 6 to 12" ruler is good as a straightedge also.

If you don't have a machinist square, HarborFreight.com sometimes has a nice three-piece set of 2", 3" and 4" squares for less than $8, and they are quite good and accurate - an essential modeling tool. Get their 6" digital calipers often on sale for $10-20 as well; you'll be glad you did. Once you use them you'll never know how you managed without them. They also have needle files and other useful modeling tools as well for fair prices.

I could do an entire article on how to prepare your castings, but let me throw out a few of the best tips I know: