bondo definitely won't work. an old autobody trick to remove it is to heat up the backside of a panel with a torch and it falls right off.
i dunno the temperature an exhust gets, but my leg tells me it's a lot more than 180.
the stock black exhaust will burn off all the paint in the first few days, and hi temp exhaust paint burns off as well.
either live with it, learn how to weld better, or get some exhaust wrap...