So today I scrubbed and I scrubbed and I scrubbed to get either red algae or rust off the inside of the bowl of my bird bath... I used comet, then Bar Tenders Friend,and there is still a little left. It is made of concrete, so not sure what else I could try????? The birds have scoffed at it this year, lol. I got most of it off, but there is still a little red there... Any suggestions anyone?