Fill the tub with hot, really hot, water. Put a portion of the rim into the hot water. Wait a minute or two and then, using a wooden scraper or soft brass brush, clean the debris away. My guess is that it will come off quite easily. I have used heat sources, gently sources, not powerful heat guns or open flame, to soften crusty handle bar tape, stickers and tubular tire glue. Give it a wee try. Let me know if it works in this application.
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"98% of the bikes I buy are projects".