OK, I'm back finally, lol. Sorry for the delay but I had to pop one of these apart, make up some items I've been meaning to that ease the process, and take pics of it all. No worries, it's something I've been meaning to do, just needed enough of a reason.
So, step one employs a tip from gggGary and that is to install a large hose clamp around the metal ring. This will support it as you pry it open, but you need to pad it somehow so it doesn't scratch up the ring. There's also usually a couple little "bumps" on the inside where the screw is. Those need to be padded or they'll dimple the side of the metal ring when you tighten the clamp. Now, Gary's "solution" was to wrap the ring with some electrical tape. This didn't work so well for me on the first few I did. First off, removing the tape tended to "freckle" the paint, the tape glue pulling specs of paint off. And second, more importantly too, it didn't protect the ring from being dimpled by the clamp bumps. Now, Gary did recommend finding and using a clamp with no "bumps", but that is difficult I think. I'm pretty sure most are made with the bumps. So, my "solution" was to cut a strip of plastic from an old food container to use as padding and protection .....
This worked very well, totally eliminating the paint damage issue and nearly eliminating the "bump" dimpling one. The container I cut it out of wasn't quite a big enough diameter to go all the way around but that was OK. I was able to cut another short piece to fill in where the screw "bumps" were .....
Now you can install the hose clamp. Keep it's top edge maybe 3/16" higher than the folded area of the metal ring so you can pry against it. So, to start prying the ring open, we get to the next tip, from Mailman, and that's to use a plain old paint can opener .....
I filed the tip on mine a little bit to make it sharper and easier to insert under the metal rim's lip. This will open the lip part way, not enough to remove the ring but enough to move onto the next step. Go all the way around with this "tool" .....
And now the final step - squeeze the lip open more with pliers. Now Gary sacrificed an old pair and ground the jaws all up. I found I had some that worked just fine with no mods, these little smooth flat nose style pliers .....
This is the big reason you need to place the hose clamp with it's upper edge above the metal ring, so you can squeeze the pliers against it. Here's the "money shot", lol .....
For this you needn't go all the way around, 2/3 to 3/4 of the way will be enough. Now you can remove the hose clamp and pop the metal ring off.