Dried battery acid in tray

I have noticed some orange residue in the battery tray. The PO told me that he had an issue with the old battery and the car dying on him, I am thinking this may be from that. I scrubbed away some of the surface orange/rust and there is still another layer of it. Is the best way to clean this to sand it down to metal then re-spray? Or is there a better way to remove everything?