Inspiration
Many of us have been inspired by the latest release of the Mad Max series, Fury Road. Though I haven't seen it, the pull is hard to resist. Add this to some early work completed by Dave Taylor (www.davetaylorminiatures.com), I couldn't resist but delve into some of this myself and start a Facebook group (Hot Wheels -> Car Wars) for us to discuss the works.
If you are looking for additional inspiration, I highly recommend picking up a copy of The Art of Mad Max: Fury Road which has great layouts and descriptions of all of the work. There is also a Mad Max: Fury Road Inspired Artists book as well, which is the works of 65 comic book artists based on Fury Road. This has more interpretations of characters, environment, and landscape.
Environment
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Supplies and Tools
Cars
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When that fails, head to eBay or Craigslist. You can usually find collections up for grabs from parents who are done with them. There are also lots available from yard sales, so go cruising on some Saturday and Sunday morning with the blue hairs. If you want motorcycles, the Hot Wheels and Matchbox won't work at all. Micromachines works perfectly though. Word of warning, Micromachine motorcycles with side cars seem to be running about $13 each on eBay, so you have to really want to add one.
Tools
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drill press. Since the car bodies are made of steel, you will need plenty of cutting wheels, grinding wheels and a tungsten carbide cutter.
To supplement these tools, You will also want to have a good range of X-acto knives/blades, good jewelers saw, pliers and some other cutting tools. I also use a set of stainless steel dental picks to mold green stuff and make add on parts in green stuff.
Other supplies
Of course you will needs lots of guns, flame throwers, rocket launchers, rams, spikes, body armor, etc. if you are going to survive. To get these I started by scavenging my Games Workshop Imperial Guard (Astra Militarum) and Chaos Space Marine bits. If you are looking for bits, you can get them from lots of suppliers and eBay, but that is an expensive way to get started from scratch. These give a very heroic scale to the cars. Flames of War bits also work well and are mostly appropriately scaled to work with the cars.
For all of this of course, you can create your own parts and pieces using plasticard and green stuff. I use Evergreen Scale Models brand, which you should be able to find at your FLGS. I use Gale Force Nine materials, but you can also use grey stuff, Bees putty or other modeling materials. If you are using to bond two materials, make sure you have clamps or to hold until completely cured. For plastic parts, I use Kragle (Krazy Glue) or Gorilla Glue and to bond metal parts together, I use Testors cement.
This should get you started nicely into the Car Wars environment. I'll be following up with additional blog posts on other cars mods. To keep you inspired: