I have 2 spare tanks. One is the original, the other a rusty one I picked up off the 'bay a while back. I've used POR15 on the tank that's on the bike, and had left over metal prep. So I have looked into different methods and came up with this process... I used Caustic soda to degrease, then Spirits of salts to clean the metal. Thereafter metal prep to convert what's left over as inbetween stages flash rust appears no matter how fast I dry it. I've done one spare and the worst one is currently degreasing. More pics of the second one inside to follow as it's much worse.
