Even if you sprayed the outside, the inside will still rust. Why the inside? Well, there's a reason for the refielf hole because condensation will ALWAYS be inside the muffler and therefore is the first place that will rust. i think you'd be wasting your time spraying the case, but I suppose it can't hurt any. it'll certainly slow down any rust from salt if you have that problem?

 

cherry bombs are on amazon for 60 bucks all the time....thats my next can of choice along with a new y pipe....

 

When i got my new exh I sprayed everything with high temp paint. Now granted we dont have either snow or ice around here but like DirtyDog said, there is always condensation inside the exh system