If its an offset with an uninsulated firebox there aint any paint out there which I have happen to stumble over which will stick for very long on the firebox. This includes engine paint..stove paint etc. Best strategy for the firebox is to keep it oiled up and seasoned like a cast iron skillet. Meaning swab it down wth oil..PAM or even know a few folks who claim to use WD 40 on there. Swab or squirt it down while its still warm but not hot after you get done cooking..after rainstorms if it sets outside..and sometimes just do it when you aint got nothing mo betta to do. It should over time turn a shade of black (or blue if you use WD 40 or that whut the fella said anyway). On the rest of the pit cookchamber etc..you can paint it with high temp Stove Paint. I have some 1200 degree Dupont stove paint that sticks purty good to them parts..or you could try the engine paint if you can find it cheaper or whutever. Hope this helps. Just my dos centavos of course
Now Dave Klose supposed to have some paint they use on the space shuttle which might stick on the firebox but I think it purty expensive.