As already mentioned, grind the inside with a wire wheel down to bare metal. Then just run a few foodless cooks with a hot fire to season the inside. Spray with cooking oil before hand and let cook for 3-4 hours. Repeat 2 or 3 times and you oughta be good.
18.5" OTS, 22.5" OTG, CB Silver(Rip), Vintage Coleman Bullet, Pit Barrel, UDS