I hope you all realize by now that what is called "duct tape" is not very useful for ducts because it ages and falls apart so quickly. The only really good duct tape is aluminum duct tape. But how many of you have struggled with how to get the backing paper off, or at least, how to get it started?
When you tear the tape, note that on one side of the tear, the tape and the backing are closely stuck together, making it just about impossible to separate the two. However, on the other side of the tear, the aluminum and the paper backing will be nicely opened up between the two. So the key is to tear in the direction that leaves the open side where you want it. I call this "tearing towards the aluminum".
Once you get it started, do not pull all the backing off. If you do the aluminum will coil up and stick permanently to itself, and you start with a new piece. The best way to work is to pull the backing off a few inches, get the aluminum stuck where you want it stuck and then work away from this end, removing the backing as you go so that you never leave a long floppy sticky piece of aluminum floating around looking for something to get stuck onto.
Do it right and you will go to love aluminum duct tape. It is not strong stuff, but it does stick very well, it does stop air leaks and it does last forever.