Suction cups stick to very smooth surfaces like glass, glossy tiles or varnished wood. Even matte bathroom tiles, it should be noted, will negatively effect their adhesive strength.
In order to attain a good amount of adhesive strength, both the intended surface and the suction cup should be cleaned with glass cleaner to remove any grease or other impurities. The suction cups should be pressed into place after the surface and cups themselves are clean and dry. Contrary to the supposed common knowledge, suction cups should NOT be applied when wet.
When correctly applied, the suction of the cups should hold for 4 5 years. This is not affected by the yellowing of the soft PVC, which begins after about 2 years..
If transparent suction cups are attached to an autos windscreen, it can be that under the influence of intense sunshine a burning-glass effect will result which can burn a hole in the dashboard. This undesirable side effect can be avoided by simply employing the opaque versions of the cups.
Also interesting: the most efficient retention force proportionate to size is found with the suction cups with a diameter of 40 mm. If the air remaining under a suction cup expands as a result of, for example, being heated, the smaller suction cups will lift off of the surface. In the case of the 40 mm suction cups, however, their surface area is large enough that the suction will hold. This is especially noteworthy in the case of display windows and windscreens which are exposed to a great many temperature swings.