The Case of the Cleaning Compounds
Dear Glass Detective,
When cleaning the interior of an automobile windshield that is fogging, is it recommended to use an abrasive step in the cleaning process? Most of the car care “how-to” videos and postings online all seem to include an initial “abrasive” step which uses a nylon or polymer scouring pad, or even #0000 steel wool. I presume this abrasive step is used to physically break-up the layer of filth before using more traditional cleaning agents such as automotive glass cleaner and microfiber cloths to remove it.
I’m unsure of the hardness of automotive windshield laminated glass and if any abrasives are appropriate as I don’t want to risk scratching the glass. I am almost certain there is no metalized or high-tech coating to be concerned about on the 15-year old vehicle that I am working on. Do you have a recommendation for this cleaning application?
Thank you for reaching out to the Glass Detective regarding your question about whether or not using abrasive pads are an appropriate first step for cleaning auto glass. Your point is well-taken that many of the how-to videos and blog posts that you can find on the internet will show that this is the first step for breaking up and removing a layer of grime from a windshield.
You also ask a very thoughtful question—is using an abrasive step appropriate? After all, glass is only but so tough. Take a look at just about anyone’s phone screen (made of Gorilla Glass) and you are bound to find scratches on it. Windshields also commonly accumulate pits and knicks after years of abuse from road debris. This evidence shows that they aren’t impervious to damage from hard objects.
So just how hard is a windshield? To describe the hardness and compare it to other materials, we need a quick science lesson. The most common scientific way to measure the hardness of a specimen is using the Moh’s Scale. This scale measures the hardness of minerals on a rating of 1 through 10; 1 being the softest and 10 being the hardest. For example, talc is rated as 1 and diamonds are rated at 10. Scratch tests are used to compare two objects against each other. If one object scratches another object, then the object that became scratched is softer than the object that did the scratching, which is harder.
On the Moh’s scale, glass receives a rating of 5.5. Steel receives a lower rating of between 4 and 4.5. Therefore, steel wool, in theory, will not scratch glass. Green scrubbing pads are made from polyester, which is even softer. Therefore these scrubbing pads, in theory, will not scratch glass either. However, just because these materials can be used doesn’t mean they should be used. Let me explain.
When cleaning anything, you always want to start by using the least aggressive option possible. This significantly uses the risk of damaging whatever you’re cleaning. For this specific reason, we disagree with “how to clean auto glass” guides that recommend starting with steel wool, which is very aggressive.
In the case of auto glass, always start with a high-quality automotive glass cleaner and a clean microfiber cloth. For optimal results, follow the steps in our video tutorial on how to properly clean a windshield. Cleaning the glass first helps to remove any contaminants that could scratch the glass such as sand. You’ll want to do this before moving on to techniques that require more aggressive scrubbing.
Once you’ve removed all loose dirt and surface debris with the glass cleaner and microfiber cloth, it’s time to see if it did the job. If there is still grime on the glass, you’ll want to step up to a polishing compound that’s specifically formulated to be used on auto glass. These polishing compounds contain light abrasives that are strong enough to remove most contaminants, but still gentle enough to be safe on auto glass. These compounds usually work well for removing built-up “film” that drivers sometimes see on glass that can be caused by oils. I believe that is the specific issue that you described with your vehicle.
If the polishing compound is still not strong enough, then it’s time to move on to a scrubbing pad. We don’t recommend ever using a product on glass that wasn’t specifically made for glass, so be sure to choose a scrubbing pad labeled as safe for use on glass. These are typically made of polyester and similar (but not identical) to the “green pad” that is commonly found in many households.
Do not use steel wool. Numerous problems are encountered when using steel wool. First, you cannot be completely sure that no other metals that contaminated the steel during the manufacturing process. If a metal with a rating of greater than 5.5 on the Moh’s scale were to be mixed in, then you risk scratching the glass. Another factor to consider is that steel rusts. Unless the steel wool is new and has been kept dry, there is a risk that it has begun to rust. Rust will scratch glass.
When using any product, always follow the manufacturer’s guidelines and do not use a product for a purpose other than that which was intended. Always spot-test the product as well, to check if you might encounter any undesired effects. A great reason for this is that coatings and films that have been applied to glass can sometimes be hard to detect. They usually aren’t as hard or as durable as glass either and therefore will be more easily damaged by abrasive cleaners or scrubbers. Contacting the vehicle manufacturer or a local dealership may be useful in finding out whether a film or coating has been applied to your factory windows. Keep in mind that in the case of used vehicles, films and coatings are a popular add-on product, so you may not be able to rely upon information from the manufacturer.
In summary, to answer your original question, scrubbing pads that are marked safe for use on glass by the manufacturer can be used to clean auto glass. These can be a great solution for breaking up heavy grime! However, we do not recommend using steel wool because of the dangers stemming from contaminants. I hope this information was useful to you and I thank you for contacting the Glass Detective.