Piston Slap: Knife to know you, windshield stain

Matt Fink

Ed writes: 

I have whitish deposits on the side windows of my car that resemble dishwasher spots. Glass cleaner and brief applications of white vinegar and CLR have failed to shift them. What is your recommended fix?

Sajeev answers: 

While this question was submitted before our Sweating the Details segment on glass cleaning, I reckon the solution here needs to be more aggressive. The deposits presented here are more like paint overspray that caught the wind and landed on your ride. But what’s on there doesn’t matter, as we are gonna remove it with these items.

  1. Gasket scraping tool (that uses a utility blade, like this one)
  2. Light lubricant for use with #1 (like soap and water)
  3. A mild chemical cleaner (like MD-40 or brake cleaner) if #2 fails
  4. Carburetor cleaner if #3 fails (make sure to wear gloves)

I’ve dealt with baked-on overspray on my garage windows, and nothing worked until I went to #4. Carburetor cleaner evaporates pretty quickly, so frequent application in small areas around the utility blade is the key. I went through half a can getting overspray off my garage’s windows, but the end result was worth the effort. And wow, that utility blade sure picked up a ton of trash off the glass!

Just remember that utility blades and glass are perfectly safe together, provided there’s lubrication and the utility blade stays parallel to the glass to prevent scratching. And yes, it needs to be parallel even as the glass changes its shape on different parts of a vehicle. The more viscous the lubricant (i.e. don’t use only water, at least until you get the hang of it) the more impossible harder it is to scratch the glass.

Best of luck, but I doubt you’ll need it!

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    I believe Sajeev has give good advice here. Yes, you will often have to move up to #3 on the list, depending on what the spots actually are comprised of, but I recommend starting w/ #2 and working your way up only if needed. Things like carb cleaner can be quite destructive to your paint, decals, etc. if you happen to get a little wild with the application (speaking from experience here), so if you don’t need to get out the big guns – don’t. But it is pretty remarkable what you can get off glass with this method when you take a little time and care using it!

    “Goo Gone” works with getting off any sticker or glue. I’ve used it for getting off numerous sticky substances off my car and other surfaces. It leaves a slightly greasy residue behind that can easily be wiped off.

    Thanks Sajeev – this helps me too as the 1995 Toyota Celsior I bought last summer has some of the toughest water spots on the glass that I’ve ever seen on a car. I’ve tried razor blades to no avail and was heading toward sand paper.
    I had previously spent literal hours using a polisher to try to get through it with only moderate success and it left one window looking kinda funky. I’ll try step 4 next.

    Not a single imperfection in the resprayed paint oddly enough.

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