So the heat shield that I won on eBay for $50 looks really nice on the outside (except for a few tiny chrome blisters?) but the inside was really rusty so I soaked it in vinegar for almost a week and then scrubbed with low grit steel wool and what a difference! The tank came out looking really nice after the vinegar treatment but I left it alone overnight and now the surface rust on the inside is back. Same thing with the heat shield. the clean side was shiny just shortly before taking this pic. What can I do about the reoccurring surface rust?