The propqne tanks can be re-certified, the valve is changed, the tank is X-rayed for flaws and thin spots, if it passes the x-ray test then the tank is pressured up to 600PSI, it then sits for 48 hours and if the pressure doesn't drop a new certification number is stamped in the tank. The outside of the tank is sand blasted using glass beads as they don't remove metal - just paint and surface rust. The tank is then painted using epoxy based paints for durability. One of my companies does this, we do about 60 tanks a week.