Clean the outside of stained terra cotta pots?

Vienna, VA(Zone 7a)

I have two large unsealed and unpainted terra cotta planters that have spent some time outside and now appear to be stained in some way. The stains look like wet dirt (see photo). I have tried scrubbing the outside (as gently as I dare); the stains fade when the pot surface is wet, but return once it's dry again.

What is this? Is water seeping from the inside of the pot? I do have large plants in there, but I have a wetness probe and the pots aren't wet on the inside -- water can also drain out the bottom, so it's not pooling in there as far as I know. Is it mold or something? I've used Formula 409 and a scrub sponge on it (again, trying to be gentle). I'm worried that the terra cotta is porous and could somehow absorb chemicals so I'm trying to be cautious, but I'd love to get the stains off!

Any ideas on how to remove these stains?

Thumbnail by betsy_VA
Dover, PA(Zone 6b)

Hey Betsy, I've noticed some similar stains on some of my pots. Not really sure where they came from but greasy was what came to mind. I don't clean my terracotta pots often mostly I don't mind the stains. I just give them a brushing and a dip in bleach to disinfect. I found this website http://www.howtocleanstuff.net/strategies/how-to-clean-terra-cotta-pots/ that might be helpful. They suggested starting with a baking soda and water paste.

Richmond, TX(Zone 9a)

the "greasiness" could have something to do w/ uneven firing and cleaning will not help. I love your pots, you could still paint them terracotta. Two coats and it will seal all that stuff. Even better, if you paint with kilz, THEN apply two coats of TC color, you're set. But I hear what a big task it is to move them around. Probably best to get the light ones.

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