Tony:
Can't go without commenting on your concerns about rust "movement". The ONLY way rust can occur, is if oxygen comes in contact with steel. It won't migrate from one surface to another unless there was a crack or fault in the metal that permitted the chemical reaction that rust truly is.
What we mean when recommending ZR can be applied over rust, is AFTER the loose and/or powdery rust has been dealt with. To do otherwise is to invite the entrapment of oxygen within the "rust" structure, thus wiping out your investment, time, & energy. We all know that a white metal blast is without question the best prep, but it ain't always practical or possible, thus the stated recommendation.
HTH
WK Irish
