If your window clings won't stick anymore, they're not defective, they just need to be cleaned. Vinyl window cling material tends to collect dust on the back once it's been removed from a surface. Even a small amount of dust prevents the vinyl from sticking to anything, but once it's been cleaned, it's as good as new once again.
Window Cling Requirements
In order for window clings to stick to a surface such as glass, the surface must be completely smooth and clean. No matter how new or clean your window cling decorations are, they won't stick to a surface that's dusty, such as the outside of a house window that hasn't been cleaned in a while. Clings also adhere best if they're smoothed down so there are no ripples or air pockets between the cling and the window. For a large cling, use a plastic scraper or squeegee-style device to smooth out the vinyl, working your way from the center outwards.
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Some window clings stick to virtually any surface, so long as that surface is smooth, non-porous and clean. Windows, mirrors, filing cabinets and plastic storage bins are just a few places a quality vinyl cling adheres to in the right conditions.
Cleaning Window Cling Decorations
Put a little warm water in the sink or a bowl, along with a squirt of dish soap. Add the clings, then swish the water around a bit. If the clings are visibly dirty, rub the dirt off with a paper towel or a cleaning cloth. Rinse the clings in clean warm water, gently shake off the excess water, then set them face down on a towel until they're dry.
If they're exceptionally dirty, as may be the case if you've used them outside, let them soak in the soapy water a bit before wiping and rinsing them. Add more soap if necessary. If they feel somewhat greasy, wipe both sides down with white vinegar with a paper towel or a lint-free cloth. Vinegar cuts grease well and helps restore the clinging properties of the vinyl. Rinse them off and air dry them as usual.
If you aren't going to use the clings for a while, stick them onto a piece of wax paper or similar surface to keep their clinging surface clean until you use them again. Make sure the pieces are smoothed down, flat against the wax paper.
If They Still Don't Stick
Some window clings work best if the surface is slightly wet. For clings larger than 12 inches, DPS Banners recommends applying them using moisture, as they'll have a better chance of adhering. Add a little dish soap to a spray bottle of cool water, replace the lid and shake the solution. Spray the smooth surface where you want the cling to stick, then carefully slide the cling into place, smoothing the cling out to remove all air bubbles trapped behind it.
To reposition the cling, Signs.com recommends spraying the new area with the soapy solution, then simply lifting the cling and putting it in its new location. If you prefer, spray the back of the cling instead of spraying the smooth surface.