A rubber chair leg protector. If you buy one you can carve out the center to avoid touching the front element, and carve a matching bevel to the trim ring. Use an old toothbrush to clean grit out of the filter threads. Make sure they actually take a filter - if the front of the lens is oval, you'll never get the ring off. If you can get drops of penetrating oil into the filter threads without getting any on the trim ring, it can help. You can always wipe it off after the trim ring comes off. This photo shows some other desparate moves: using a flat blade small screwdriver in a vertical engraved line in the name or serial number, which slips, or drilling a hole, which looks bad.