Hold up before you toss those discs. That 3 hour savings might just be luck on one joint, not a real pattern. Flap wheels burn through quicker on thick plate if the steel has any hard spots, and when they load up you're stopping to clean them constantly, which eats your time right back. Plus those cheap discs are predictable, you know exactly how they're gonna act on every pass. I've seen guys chase the speed with flap wheels and end up redoing the bevel because the heat warped the edge. Sometimes slow and steady still wins when you're talking about weld prep on anything over 3/4 inch.

Lost a Canonet to this exact thing last summer. Silicone spray on a q-tip, dab it along the rail where the curtain meets the metal, not the cloth itself. Let it sit for an hour, then work the shutter with the body cap on and the lens off. That freed mine up without touching the fabric at all. If the curtain is already frayed at the edge, you're looking at a reline job anyway, so don't stress about making it worse, just get it moving first.