I know it sounds nuts but I agree with the Pbutter treatment. Apply a liberal coating to soften it up (in the shade). Leave it on for at least 30 minutes. Clean it off with Simple Green and a light brush. Good luck
AHHHH p-nut butter, the universal solvent. In the breakroom where I worked someone tried to use liquid nails to attach a plastic sign to the ceramic tile over the sink. It didn't hold the sign at all, but stuck really well to the ceramic. The assistant manager brought in a small jar of PB, smeared it on, and everyone thought he was nuts. It took 2-3 applications but it did remove the liquid nails.
Strange but true, don't know why it works, it's a handy thing for yer bag of tricks.