Batteries have been known to bubble over and I have had this problem in my airplane. You want to first neutralize the acid with a water baking soda mix or one of the spray neutralizers available in the auto parts store. Then paint the metal with one of the paints designed for battery boxes. These paints aren't affected by the battery acid if you should have this problem again. I would check the voltage generated by alternator to see if it is too high. If the battery has any vent hoses, make sure the hose hasn't been disconnected. Could also be a bad battery. If it's still under warranty, try that first.