What Causes Suction Cup Marks on Glass
If you’ve ever noticed faint circular marks appear on your windows during humid or foggy conditions, you’re not alone! These marks, which are caused by suction cups, are a natural result of window manufacturing and installation practices. While they may occasionally be visible, they have no impact on window performance or longevity.
At Viwinco and Window Manufacturers around the world, suction cups are used on automated assembly lines to transfer glass lites through the manufacturing process. These marks do not represent damaged or defective glass. The suction cups don’t change the glass— they change the way water beads on the surface of the glass in these areas. Because of this, suction cup marks are not covered under warranty and it’s important to understand why these marks occur, and how to remove them.
What Causes Suction Cup Marks?
The appearance of suction cup marks comes down to how glass interacts with pressure, surface energy, and condensation.
Surface Energy & Contact Angle Changes
Glass has a relatively high surface energy, so water tends to spread on it. When a suction cup presses against the glass, it creates tiny, microscopic changes to the surface, sometimes leaving behind tiny traces of oils or residue. These small variations alter how water interacts with the glass.
Nucleation of Condensation
On a humid day, condensation forms when warm air hits cool glass. Humidity and condensation is likely to highlight areas of the surface of the glass that have these marks, or any others. The suction cup’s contact point becomes a preferred spot for condensation to nucleate- meaning it’s an area where water droplets first begin to form and collect which makes the circle visible.
Optical Contrast
Because water droplets form differently on the suction cup area versus the untouched glass, the area becomes noticeable when there’s fog or condensation. Suction cup marks are the result of microscopic surface changes that affect how condensation develops, not a flaw in the glass itself.
How to Remove Suction Cup Marks
While suction cup marks don’t affect the performance of your windows, some homeowners may find them distracting. Luckily, these marks can be removed with 0000-grade steel wool, which gently polishes the glass and does not scratch it. This process works by buffing away microscopic residues to restore a consistent glass texture, physically removing any subtle variations left behind by the suction cups. Follow the simple steps below to refresh your windows.
Materials Needed:
• 0000-grade (very fine) steel wool
• Glass cleaner or soapy water (spray bottle preferred)
• Microfiber cloth or paper towels
1. Clean the Glass: Start by washing the glass using soap and water. This helps to clear away any dirt or debris that could scratch the surface when you begin to polish.
2. Prepare the Glass: Lightly mist the glass with a glass cleaner or soapy water. Keeping the surface of the glass wet will make the polishing process smoother, more effective, and will prevent the 0000-grade steel wool from scratching the glass.
3. Polish with Steel Wool: Take your 0000-grade steel wool and gently rub over the marks using a back and forth, or up and down motion. Don’t worry, this grade is safe for glass and won’t scratch when used properly as long as you keep the area damp as you work.
4. Clean, Rinse, and Dry: Once the marks are gone, wipe down the glass with a microfiber cloth or paper towel. Rinse with clean water if needed and enjoy your clear, streak-free window!
Suction cup marks are a normal part of glass manufacturing, and appear only under specific conditions like fogging or condensation. They have no impact on your window’s performance or durability, and with a simple cleaning technique- they can be removed entirely.
At Viwinco, we take pride in producing high-quality windows and want our customers to feel confident in both their performance and appearance. If you have any questions about cleaning the suction cup marks off of your Viwinco glass, our customer service team is here to help. For troubleshooting, maintenance tips, or product support, contact our customer service at 800-927-2848.
