Magic Melting Marks – Lines on Your Rear Windshield Explained

Defroster in your back window:  Have you ever wondered how those little lines on your rear windshield defrost so quickly and easily? Well, I have too which is why I decided to do a little research to find out what they are, how they work, and why on Earth they aren’t on our front windshield.

What are they?

So what are these lines made of anyways? Well, as it turns out these lines are metallic paint that rests atop the inside of the glass so they don’t get chipped off as easily. Technically they are a high concentration of metal (most likely not led with the new regulations) and adhesive. Alright, we know what they are but why the back and not the front? Actually, there are many reasons they primarily exist on your rear windshield.

But why not the front too?

Probably the main reason only the back windshield has them is that the front has air blowers to defrost while the back rarely does. The front blowers are also much cheaper than the metallic lines as heating ducts will always be incorporated into the car. This is why it costs so much more to replace a rear windshield than a front one. The factory has to use more resource and must conduct more tests to make sure the defroster lines work.

Another major reason for no lines on the front shield is visibility requirements set in place by the Department of Transportation of each state. Most states require no visual obstructions on the main portion of the glass and therefore prohibit defrosting lines to be painted on or implanted in the shield. Now you may be thinking how dumb of a rule this is since I could shave a bunch of time off defrosting my windshield but hold on. It can, in fact, be done and has been done before!

Not exclusively in the rear

It is actually not that uncommon to find thin metal wires implanted in the windshield in a sort of a zig-zag formation. They are thin enough to not be too much of a visual obstruction but just large enough to get the job done. These beautiful wires, just like the metal paint lines, begin working as soon as the car is on and the defrost is engaged. There’s no need to wait for the car to heat up at all. Just poke the button and BOOM your windshield is defrosted in no time. It really isn’t fair this isn’t a thing in America but, maybe someday it will be. Actually, some newer cars have the defrost lines painted around and underneath the windshield wiper blades to prevent them from freezing and damaging the motor when turned on. Still, full windshield defrost-lines would be pretty sweet.

How do they work?

Now you know what they are and where they can be placed, it might be nice to know how they work. As they are made of a metallic paint, the system sends currents of electric throughout the lines to create electrical friction. This friction generates heat and quickly defrosts the surrounding area.

The coolest part is that these lines most often feel cool to the touch when turned on. The reason behind this? To defrost the glass, the temperature only needs to be raised slightly above freezing to melt ice or snow. Pretty neat right? Besides if they got hot enough to melt or burn anything that touches it, your engine would have to divert too much power for the whole process to be worth it. This is why most cars will shut off the wires far before they ever get too warm.

However, since these lines are just one large electrical circuit, the connection always needs to be complete, which means that if any section of wire paint is chipped off, the circuit is broken, and the defrost lines will be ineffective. Lucky for you, there is a fairly cheap and simple method of repairing the lines, rather than replacing the entire glass which is far more expensive than a regular windshield. All you need is the conductive metallic paint.

There are a multitude of brands and kits you can purchase online which include all the paint, adhesives, tapes, brushes, and everything else you would need; some even include am ohmmeter to test to see if the circuit is complete. Most kits are around $20 but there are some cheaper and more expensive ones out there.

There you have it

So now you know. For the curious or bored, the self-taught or fact finders, you now know what the magical little lines are and what they do. You know why Americans only ever see them on the back glass and what you need to fix them if they ever stop working; God forbid they ever do.

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Author: B. Delamater

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