Some window decals, such as the type called static decals or window clings, are designed for temporary use, which also means they're easy to remove and easy to reuse. These have no actual adhesives on them, so there's no gummy residue left behind on your windows once you take them down. Window decals that have adhesives on one side are generally not reusable, and they're more difficult to remove from glass. Since the terminology varies a little from one product to another, make sure you're getting a reusable static cling variety if you intend to reuse it.
Window Clings vs. Decals
Window clings use static electricity to stay in place and nothing more. They'll adhere to virtually any smooth, nonporous surface, such as glass or metal, but the surface must be entirely free from dust. A cling may be made to go inside so the design faces out, or it may be placed on the outside of the glass. If you're planning to leave one up for a while, it's best to place it inside, as static-based window clings are not weather-resistant and could come off.
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Window decals are semipermanent and have an adhesive on one side. They're made of a weatherproof vinyl, so they'll work well on the outside of a window, as is the case with vinyl window stickers used as small decorations on a car's back window. Window decals are a lot like stickers or vinyl bumper stickers in that once they're smoothed into place, you may be able to remove them without much trouble right away, but the longer you wait, the harder it is to take them off. In many cases, soapy water, a razor blade, and/or a heat gun may be needed to fully remove them and their adhesive. They generally cannot be reused.
Removing and Reusing Window Clings
Removing a window cling is as easy as picking at one corner of it and then pulling the entire vinyl cling off the surface. To save them for reuse, simply smooth them down onto the shiny side of a sheet of used sticker backing paper or a sheet of waxed paper. Reuse them by cleaning the window and then smoothing out the clings onto the window surface using a plastic gift card to remove air bubbles.
If the cling won't stick well, wash it in warm, soapy water and try again. Larger clings stay on best if they are put in place while the glass is wet. Spray the glass with a spray bottle filled with tap water and a drop of dish soap and then smooth the cling over the wet surface. Smooth out the cling with your hands and then use a plastic scraper or gift card to remove all air bubbles. If the cling is so large that your scraping device drags a bit across it, wet the front of the cling as well. Wipe up drips afterward.