Typically the rust/corrosion is from leaking batteries.
I think you did the right thing in priming it. It's a lot better than to have a body shop cut the tray out and welding in a new one.
Makes sure your drain tube is installed on the battery. Battery water level is always properly filled, not too low or too high. And just keep and eye on the battery, check it at every oil change (3k not 10k)