residue in cooler after soaking bottles.

I left some bottles in my cooler to soak overnight and neglected them for a week. Now the cooler has a brown/beige hard residue that a hot oxyclean soak and cloth scrub couldnt clean up.
I want to use this cooler to mash, but not going to do it till this residue is gone.
The bottles had the previous homebrewer's labels on them, so no telling what kind of glue it is. . . Assuming it is even glue.
Anybody had residue in the container after a long soak?

Try an acid cleaner like vineagar. Is the residue just at the water line? Could it be Iron or a hard water deposit?

i bet its ink from the labels, staining the plastic.
+1 vote for an acid cleaner. Star-san soak might work well.

Didn't even think about ink from an inkjet printer!!! If acid does not work, bleach should at least remove the staining.

If StarSan won't take it off, there is always this: