i think there is a mis-conception that porcelain coating just lasts longer. My off-set is painted with 1200 degree paint and I got some cans of Rustoleum high heat just waiting......fireboxes just destroy the paint.....almost after the first cook. I read somewhere to let the firebox paint burn off and just coat the firebox with oil and treat it like a big old cast iron skillet.....
Here is a little cleaning trick for the Weber or PBC......after the cook when they are warm to the touch, go over them with a slightly damp rag......they look like they just come out of the box when I do that....................