This thread has been very helpful for me. I have tried getting some very old soap scum off of a cast iron tub for years. I have used just about every cleaner out there, but the Dawn & Vinegar is slowly, but surely working. This is the first time I saw any progress. I spray, let it sit for at least 30 minutes, then scrub off with an extra toilet brush and rinse. I am beginning to see the real color of the tub again. Still a work in progress, but so much better. Thanks for all the info.