This is a bit of a random question, but what is the size of the bolts holding the fender in place? I am replacing the dented one on my car with another and I keep twisting the heads off the 27 year old bolts.

 

Mazda used 6mm hex head taptite bolts with a yellow / gold passivated finish to secure many items, including the front fenders to the shell. Taptites cut their own threads into the captives previously welded into the shell and save the expense of tapping threads 1st at the factory.
I buy them by the box, very cheap from my local supplier.
Frozen fixings are a pain where working on old cars so I change mine whenever I renew a part... put a bit of copper grease on the threads and they'll come out easy next time

 

I think the paint also helps create a sort of bonding, I remember trying to remove my fender bolts and snapping many of them.