I have been a professional window tinter in Vero Beach Florida for over twenty years. Removing old automotive window film can be hard work, but with a few tips and techniques it can be a little easier.
There are many manufacturers of window film, and all of them come off different. 3M, Solargard, Llumar, Johnson, Aegis, are all different manufacturers (and there is a myriad more). There are also many different lines by each manufacturer. For example: Solargard has Supreme, Galaxie,Quantum,HP Smoke, Ultra Performance, and even more that they carry for just automotive window films.
All of these film manufacturers and types come off differently. With so many different possible types of window film you have to start from square one to find out which tip or trick is going to work for your particular application.
Step one. The vehicle needs to be completely cool and in the shade. The best would be first thing in the morning before any sun has heated up the glass. You want to pick at a corner, and start to peel the film. You need to pay extra attention here.
There are two ply's to the window film. There is a layer of adhesive to the glass, a ply of film, another layer of adhesive, and the top ply of window film.
If both ply's do not come together... STOP! it is very important that both ply's come together only leaving glue residue behind. If this is not happening now we need to try step two.
Step two. Now we want the vehicle in direct sunlight. The hotter the better. You are going to pick at a different corner again trying to get both ply's to come together while peeling. If this attempt does not get the film to peel with both layers intact we need to move to step three.
Step three. Still in direct sunlight, and hopefully hot as this will not work with sunny and 35 degrees outside. This is for 80 plus. You need to use a black garbage bag, and cut it out to the shape of the window. You are going to spray the window with windex or even a mixture of soap and water. It really doesn't matter as long as it is wet. The place the bag over the film on the inside. The wet will make it stick, and let it sit for twenty plus minutes. Give it a check after the time, and wet it again. After another twenty plus minutes try a corner again, and see how it peels. You are basically trying to steam it like an envelope at this point.