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same happened to me, i accidently dropped some battery acid on the front fender and after few days i saw that it had blackened the paint from that area, my car's colour is dark grey.
a local painter said that acid nay paint saarrh diya hay.
any solutions to prevent it from further spreading or may be fixing it???
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@rauf_ffc
i think in your case it is caused by overfilling of acid in the battery, thats why when it charges it bubles out, as your damage is under the bonnet u can easily get that area painted.