Don't know why your battery is causing corrosion, but to prevent in the future, clean all the old paint off (as much as you can). And then apply POR-15. See here. This stuff works.

__________________
Steve

1992 300CE Sportline Sophie
1990 300E Emma - in the family
1979 240D Josephine - sold, but not forgotten
2004 Pacifica AWD

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