I've burnt rice to black carboniferous slag on the bottom of my direct-fired kettle before. After a day of serious scraping, wire-brushing, and a pile of green scrubbies, my kettle now shines after each and every brew. I don't do multiple brews in one day, however. At another brewery, I did. There I would CIP once a day after all beers were finished. And I do recall a thread about this posted somewhere.
Phillip Kelm--Palau Brewing Company Manager--
Worldwide Brewery Installations