Food grade usually means that it has something like a reddish epoxy-phenolic blend or straight phenolic lining that is really tough to get out. Can takes more time than building the UDS itself, but you gotta get it all out of there.
Not everyone has success with getting it all out by burning alone.
Jim - Another transplanted Texan
Former KCBS CBJ
Large and Medium Big Green Eggs , Black 18.5" WSM, Blue Weber Performer - Stainless, Green Weber OTG Kettle , Brinkmann SnP Pro, and a Stainless UDS. One retired Portable Kitchen grill.
Red Thermapen, Maverick ET-732, EdgePro Apex Sharpener.
Avatar is the original 1951 Weber Kettle