Unfortunately, even the highest quality window film won’t last forever, and at some stage will wear out to the point where it needs to be removed. The two most prominent signs of expired film are 1) bubbling, and 2) purpling.
Bubbling film is caused by natural deterioration of the underlying adhesive. Once you spot your first bubble, you can be sure that more will appear soon enough. The other sign – purple film – comes as a result of the breakdown of non-metallic dyes, which causes the film to turn a purple color.
The difficult thing about removing tint is that most film is constructed of two layers of tinted polyester. When you try to peel the film off the glass, the top layer will usually pull right off leaving the second layer behind. This second layer will not peel off, it will just flake and tear like a price tag.
NOTE: Some cars have radio antennas mounted alongside the defrosters lines made of the same material. If you damage these, your radio antenna will not work! Taking it to a tint shop to be done greatly increases the odds of saving the lines, but even a tint shop can not promise to save your lines, sometimes the film is just to far gone.