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Don't force it hard or you'll crack the glass or frame. Run a utility knife or old credit card along the paint seal on all four sides, then gently tap a block of wood against the frame to work it loose before trying to slide it.
The utility knife approach works, but I'd actually start with heat first - a heat gun or even a hair dryer on high will soften the paint and make it more brittle so it cracks cleanly rather than peeling away in chunks. Once you've warmed it up for a minute or so, then run your knife along the seal. The wooden block tapping is solid, but be patient with it; a few gentle taps, then let it sit for a few minutes and tap again. If it's really stuck, sometimes a bit of penetrating oil (like PB Blaster or even WD-40) worked into the crack overnight does more than anything else.
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