How to Make a Female Mold From a Car

Written by richard rowe
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How to Make a Female Mold From a Car
Stock car teams use moulds to produce entire replacement bodies. (Todd Warshaw/Getty Images Sport/Getty Images)

Whether you're building lightweight replacement panels from fibreglass or carbon fibre or building a replica of the entire car for display purposes, the entire process starts with making a mould of the panel or vehicle to be copied. This procedure may be as old as fibreglass itself, but modern tools and materials can make it easy enough that anyone familiar with a compressor and fibreglass can do it. Making a mould requires some prep work, but after that it's basically a matter of applying the right products in the right order.

Skill level:

Things you need

  • Basic hand tools
  • Duct tape
  • Cardboard
  • Wax
  • Mold release agent
  • Air compressor
  • Paint gun
  • Tooling gel
  • Fibreglass
  • Tooling resin
  • Hardener
  • Fibreglass thinner
  • Paint brushes
  • Fibreglass roller
  • Rotary tool and accessory set
  • Plastic razor blade

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  1. 1

    Remove the car's rear-view mirrors, radio antenna and bonnet ornament. If you mould around these items, you won't be able to get the mould off without destroying them in the process. If you want to replicate the rear-view mirrors, for example, you'll have far better luck moulding them, making them out of fibreglass and attaching them to the finished replica afterward.

  2. 2

    Fill any deep recesses or holes in the car from behind with cardboard or a double layer of duct tape. Slide a piece of cardboard behind the grille and bumper openings, as well as any vents in your bonnet or fenders. If you can't get tape to stick to the panels between the panel gaps, shove the tape into the gap with the sticky side facing in. This will keep the bright orange tolling gel out of places where you don't want it to be. Place a piece of tape along the bottom of your door handles and over the lock mechanisms.

  3. 3

    Apply three to four coats of carnauba wax to the paint and anything else that the moulding agent will touch (including the windows, headlights, taillights and trim). The wax will form a barrier between the mould-release agent and the paint, helping it to break suction and release more easily.

  4. 4

    Spray the entire car with four to five light coats of mould-release agent, following the package directions for mixing and spraying. When it dries, the mould-release agent forms a thick, plastic sheet over everything it touches. This sheet peels off of the car allowing you to remove the mould without damaging it or the car.

  5. 5

    Load a primer spray gun with the largest-orifice tip you have available with tooling gel. This substance is very similar to the gel-coat that goes on over any fibreglass part, but it's far thicker, more durable and pliable enough to withstand numerous castings using the same mould. Spray the entire car front to back until it's covered in a shell of orange tooling gel about 3/32-inch thick. Three to four coats will do it. Allow the gel-coat to dry for an hour.

  6. 6

    Lay your first layer of fibreglass mat over the bonnet. Add to one quart of your tooling resin the amount of hardener specified by the manufacturer. Saturate a paint brush with resin and begin applying it to the fibreglass mat. You'll know the mat is fully-saturated when it turns transparent enough that you can see the orange gel underneath. Go over the fibreglass mat with a fibreglass roller every minute or so to work the bubbles out, then store your roller in a container of thinner to keep the resin on it from hardening.

  7. 7

    Continue fiberglassing the entire car body from top to bottom and front to back, mixing the resin one quart at a time. If you mix a huge batch, not only will the resin get very hot, odds are it will begin to gel and harden before you get the last of it into the mat. Once you have the entire car covered, start over again on the bonnet where you began, applying a second and maybe even third coat of fibreglass. It's going to take you at least an hour to apply each coat, which is plenty of time for the previous coat to set up. Allow the car to sit for two to three hours when you're done.

  8. 8

    Fit a cut-off wheel to your rotary tool. Start at the front of the car and cut the mould apart all the way down the vehicle's centre line. This takes a very, very light touch. Only cut through the fibreglass itself, not the orange gel coat. Once you see orange, stop cutting in that spot and move on. Cut the entire mould in half lengthwise.

  9. 9

    Make whatever cuts you need to free the mould from the car. On some cars you might get away with only cutting the mould in half, but odds are you'll need to cut it into at least six pieces -- two more lengthwise cuts down the side of the car just above the wheel openings and another cut around the middle of the car near the back of the front door. The car's shape will determine how many pieces you'll have to cut the mould into.

  10. 10

    Cut through the gel-coat using a plastic razor blade. You'll probably need to apply some serious pressure and make multiple passes with the razor in order to fully cut the gel-coat. Take your time and make sure that you're all the way through the gel-coat and mould-release film before attempting to remove the mould.

  11. 11

    Pull up the corner of one of your mould pieces and insert the end of an air-sprayer attachment underneath it. Wrap the open area around the air fitting with a wet rag, sealing it as best you can. Trigger the fitting to inject compressed air between the mould and the car body while an assistant helps to work the mould free. The mould may just pop off the panel, but the odds are that you'll have to wiggle it free. If the mould refuses to come off, break out your rotary tool and cut it where need be.

  12. 12

    Pull all of the mould pieces off of the car one at a time. Don't be afraid to make cuts where you have to, but take your time making them; you're only going to get one chance to do it right. Don't be too surprised if you wind up with 20 or more different mould pieces by the time you get done.

Tips and warnings

  • After you've pulled your panels off, you'll need to make a "buck" to hold all the mould pieces while you're making your fibreglass body shell. Start by reassembling the puzzle-pieces of the mould back into the shape of a car. Have someone crawl underneath and tape the pieces together as you assemble them. After you get the puzzle put back together by using up to three rolls of duct tape to make sure it's strong and straight, repeat the mould-making procedure to make the buck. Spray the backs of your mould pieces with several coats of mould-release and then a thin coat of tooling gel, then lay on as many layers of fibreglass as you'd like to keep it rigid. You may also wish to incorporate some wood supports into your mould buck to make it as strong as possible.
  • To make releasing from the buck even quicker and easier, you might consider covering the backs of your mould pieces with cardboard, then fiberglassing over it. Sand the fibreglass smooth, prime and paint it as you would a car body. This additional step can save your some time and aggravation in removing really complex and curvy moulds from your support buck. Do everything right and you'll have a mould set that will turn out dozens, if not hundreds of replicas before it wears out.

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