Remove a step-up ring from a filter

Since I moved to the Sony from Canon, I’ve had to buy some step-up filter rings to use my 77mm filters on the various Sony lenses. Simple solution, and to some extent it is better having larger filters on wide angle lenses to avoid vignetting. But what when the ring has been on your polarizing filter for some time and it refuses to budge? The polarizer is quite thin, it rotates of course, and so the amount of “gripping” space is pretty small. I saw a video about tapping the edge of the filter several times to try to free up the threads, but it didn’t work for me. And then I thought back to my Physics days – expansion due to heat! So I put a thin layer of hot water (from the tap, not the kettle) in a plate and put the filter (ring side down) into the water. The level was designed to just cover the step-up ring and no more – it didn’t touch the glass. Waited a few minutes and then, bingo, the ring easily unscrewed as it got warmer and expanded away from the threads on the filter itself. One little tip to add to your memory banks!