Having cut my old exhaust off prior to fitting a new one, I thought I'd whip the heatshields off and sort out the odd blip of surface rust underneath the edges, mostly around the bolt holes.
I discovered the shield itself has been attacked by moths, or at least that's what it looks like. There seems to have been some sort of chemical reaction where the steel bolts go through the aluminium sheet. I don't want to put it back on like this as it can't be taken off again without dismantling the exhaust. Three of the four RH bolt holes are like this to some degree - oddly enough the LH holes aren't.
It if was flat I'd bolt two strips of aluminium sheet along the RH side to act as clamps - that would solve the problem. It's not flat though.