With the right blade and proper use, a hacksaw can cut through plastics and metals quickly and easily. A few of the uses you might put your hacksaw to when working on your car are: cutting a frozen U-bolt when replacing the exhaust system; trimming excess plastic off a tailight lens that doesn't quite fit; shortening a replacement bolt that's too long; making mounting brackets for items such as accessory lights.
You can buy a hacksaw at auto parts supply stores, hardware stores and other places that sell tools for $2 to $10, depending on where you shop. The main things to look for are a sturdy frame and a solid handle attached rigidly to the frame. A flexi-frame hacksaw wears out blades faster and is hard to use. The key to buying blades in the number of teeth per inch - you can get 14, 18, 24 or 32 teeth per inch. A 14-tooth blade is best for cutting metals 1 inch or more in thickness. Use an 18-tooth blade for materials 1/4 to 1 inch thick; 24-tooth for material 1/8 to 1/4 inch think and 32-tooth for materials 1/8 inch thick or thinner. For most automotive uses, the 18 or 24-tooth blade works fine.
When installing the blade, turn it so the teeth face forward. Tighten the blade until it gives a pinging sound when you tap it with a screwdriver. A loose blade will not cut well and is much more likely to break.
To start an accurate cut, let the blade rest lightly against the edge of your thumb. Draw the hacksaw toward you a few times, to make a slight indentation to guide the blade when you start on the forward stroke. After a few backward strokes, start making short forward strokes, applying moderate pressure on the hacksaw. When the blade shows no tendency to slip out of the cut, apply firm downward pressure and make longer strokes. Apply firm pressure on the forward stroke and release most of the pressure on the back stroke. A good cutting speed is 40 to 50 strokes a minute.
A hacksaw requires no special care, other than caution not to bend the frame - a bent frame won't make a straight cut. But unless you use the hacksaw as a pry bar or leave it where it can be driven over or stepped on, that won't be a problem.
Remember, whenever using a hacksaw under a car, wear goggles to protect your eyes not only from tiny bits of metal dropping from the cut, but also from the rust and other foreign matter your sawing will dislodge from the car's underbody.