"Kitbashing" is the practice of using parts from different model kits to customize a model. What you usually end up with is still recognizable as simply a variation on what the original kit was designed for. But you can go further and merge completely disparate kit parts into a thoroughly new, fantastical creation.
For instance, in the photo here you see what started out as a Jeep model -- specifically the "Wild Willy Jr." kit from Tamiya -- with the wheels replaced by the legs from a dinosaur model. I call it "Four-Leg Drive."
The kind of combination you come up with is limited only by your imagination, your preferences and the availability of different model kits that can in some way work together.
Getting Started
Picking out these kits is your first task. You'll need to keep an eye on scale, so the parts will "look right" when combined. For instance, in the "Four-Leg Drive," the legs from a substantially larger or smaller dinosaur wouldn't work very well. I chose legs from a triceratops kit that were about the right size to fit into the wheel wells of the Jeep.
Even when the parts are close to being the right size, you may need to do some cutting and filling to get them to merge seamlessly. Cut using a sprue cutter, a sharp hobby knife like X-Acto or a coping saw. Continually test-position the parts in the area where you want them to go so you'll be left with little or no gaps. Then glue them into place. Once the glue is dry, fill in gaps with putty. Err on the side of using more putty than you need, as you can file and sand away the extra when it's dry. Some kinds of putty shrink a little as they dry, so you may need to go through a few cycles of adding putty, letting it dry, sanding it down and adding more putty. Keep going until the surface is as smooth as you can get it.
You can use standard model kit paint for the finished project, but I tend to prefer acrylics. You can apply acrylics thickly, which can be handy for filling remaining seams or evening out surface texture on your first coat, or thin it down with water to add subtle color overlays on succeeding coats.
Options
One fun aspect of combining disparate kits in this way is choosing what detail pieces to include: Perhaps instead of the spare tire on the back of the Jeep, you could attach the dinosaur's tail. Or put its horns on the hood, or eyes in place of headlights.
Look through the aisles of a hobby store or in online model kit supply catalogs for inspiration. Think about things that would look cool when combined. Ideas include putting bird, bat or pteranodon wings on a jet, putting a tank turret in place of an animal's head or on the roof of a car, or mounting the upper body of a human figure onto a horse body to create a centaur.
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