There are three basic methods for removing mylar: the freeze method, the heat method and the chemical method (orange cleaner/degreaser). Each has its own pros and cons and none of the techniques can be guaranteed not to damage whatever is underneath the mylar.
I have found the best method for removing playfield mylar is typically the freeze method, which I will cover in this article. Before we start though, I cannot state strongly enough that you risk damaging your playfield artwork with this or any other technique. Although if you follow the steps exactly, you will minimize the chance of damage occurring.
The first thing we need is some damaged playfield mylar. In my case, I've got a Data East Hook Pinball with a piece of round mylar under the center kick-out hole that has a worn spot (see Image 1).
Next, we need to assemble our tools (see Image 2). You will need either a can of freeze spray or compressed air (just turn the can upside down when spraying), some Gel Goo Gone or Sprayway 909 Heavy Duty Orange Power Plus (available at pinrestore.com). You'll also need either a metal or plastic razor blade (the plastic blades are available at pinrestore.com).
See the references for a great method for removing the residual mylar adhesive from the playfield.
The first step is to lift one corner, or edge, of the mylar far enough that you can get a grip on it. You need to work the razor blade under the mylar about 3/4" to 1" or far enough that you can now grab it with your fingers (see Image 3). If you're lucky you will find a spot that is already starting to lift and you can start there. In my case the piece of mylar is round, which is the hardest to get started.
Spray the mylar with your cold spray, or upturned compressed air, wait until some ice crystals form and then wiggle the razor blade slowly from side-to-side. If you're having trouble, spray it again and wait for the ice crystals. As you can see in the attached video it takes a little time and patience to safely remove mylar and you may have to apply the cold spray several times to get the glue to break loose.
Once you've got a corner lifted, also spray under the mylar.
Now you want to slowly pull back the mylar as you spray it with the cold spray (see Image 4). An alternative method is to keep working forward using the plastic razor blade to llift the mylar.
You should not have to pull very hard. If in doubt use more cold spray or stop and try one of the other methods, it's much better to be safe than ruin a playfield.
If you are removing a large piece of mylar you need to be especially careful because it's hard to keep it cold all at once. In this case, it may work better to start at the outside in several spots and then work towards the middle, switching between sides as you proceed.
If you have an area wider than about six inches you can remove it in strips by cutting along one edge with a pair of scissors as you lift the mylar.
The older the machine, the more likely you will pull up some paint. Newer machines with clear-coated playfields go a lot easier, so don't get overconfident, and then ruin your favorite electro-mechanical pinball by rushing the process.
Once you've gotten the mylar off, squirt some Goo Gone or Sprayway on the leftover glue residue, let it set a few minutes and then rub it off with a shop towel. You may have to repeat this process a few times. If your playfield is in good shape, you could also use a plastic razor blade with the Goo-Gone.