You can try soaking the channel in goo gone, then try to slowly work it loose. The goo gone won't harm the paint (but wait a day or two to let it dry if you plan on using triple thick. The goo gone repels it a bit).
Another method is to hang it upside-down and let gravity do the work.
https://www.flippers.com/pin-tips.html#Remove_Rail
Another method is to use a block of wood and a mallet to slowly tap the channel off the glass.
Or, lastly, check with gatecrasher on the forums to see if he has a substitute channel that might fit if yours is all rusty.