How do you keep unpainted cement birdbaths clean?

use paint that is safe for birds to drink out of. The people at paint store should be able to help you. Other than that...I think elbow grease and a brush do the truck. Any chemicals can hurt the birds

I use a steel scouring pad and just rinse really well.

Just scrub out once a week and it should be fine. You can get paint made for cement but is not going to change the problem. Our had green sludge from the trees above and painting did nothing to change that.

Empty it and sprinkle baking soda on it. Then pour vinegar on it and it will cause the baking soda to kick into action and lift the slime. Science is so fasinating!

My mom had an unpainted cement bird bath. She used bleach to get rid of the black or green that scrubbing didn't get and just rinsed it with the hose real well before she refilled it. It never seemed to harm the birds, and she had a lot of them that visited her bird bath. I got a plastic one with a polished basin for that reason, so far it just needs hosing out and it is sparking again!

Thank you for the suggestion, I will try them all out!

I asked the associates at the wild bird store and they said to use a little bleach and water, scrub good then rinse well.