You want to use JB Weld on the exhaust of your car when you have a leak. Maybe you have a small leak or a cracked pipe and can’t afford to have it replaced or fixed properly. For small holes or cracks, regular gas torch welding or MIG welding is recommended, but JB Weld will also work in a pinch. You must make sure to mix the two components in the proper ratio and mix them thoroughly. You must also make sure that you use a wire brush and sandpaper so the two pieces being epoxied are as clean and rust free as possible, or it won’t work.