The brightener or acid wash actually removes an outside layer of the metal and leaves a fresh but more pitted surface behind. That in turn oxidizes faster and has you using the acid again but sooner and the process continues. Polishing with a buffer smooths the metal and removes any oxidation. Do this and add a coat of GOOD wax or shark hide polymer after and it will last much longer. Fuel tanks on rigs done this way last months but the acid might make a month and looks dull again.