Is your battery leaking? It may be caused by spilled battery acid from the years of holding batteries. I had a fair amount of rust in my battery area when I bought my Supra. To get rid of it I wiped the area down with a water/baking soda mixture to neutralize the acid (if I remember correctly, 3 parts baking soda and 1 part water), then sanded it down to bare metal, primed and painted it. I then relocated my battery to my trunk but had a major electrical problem that caused my battery to leak and low and behold, within one week I started seeing metal corrosion from the leaky battery case. I did the same there, fixed the electrical problem, and then bought an Optima sealed battery which hasn't leaked since.