I think I know the answer to this, but prove me wrong...
I recently purchased a used 3-vessel system. Overall the system is in great shape, however it was stored in a shed for a while. The kettles have some minor rust but should clean up just fine. The false bottoms are proving to be another story. The minor surface rust on them wasn't an issue, but there was some heavier rust and oxidation on the welds.
I've tried BKF, it's not cutting it. I'm assuming that any and all rust must go, but I can't seem to shift the rust from the weld spots. If it can't be fixed easily, I'll just fab up a new mount and use my SS Brewtech mash tun in it's place.