Hi Bill,
I have no idea if this works or not, but when I got a new vance and hines 4 into 1 before I fitted it I sprayed a lot of VHT paint inside it, enough that I am pretty confident it covered everything. Then every time I wash and clean the bike I spray a load of WD40 into the end of the exhaust. Smokes a bit next time i start it, but not for long
As I said I dont know if this works as the pipe has only been on for 2 years, but no rust yet!
The other thing I do, is in hte winter the bike is covered with a cheap indoor bike cover and under it is a 180 watt greenhouse heater, (one of those electric tubes) mounted on a bit of ply covered with tinfoil to reflect the heat up, this sits right under the bike. Now I do know this works superbly, MY bike is all polished alloy, without the heater it will be covered in condensation, with it on there is none at all and the alloy will stay shiny all winter with out me touching it. This means as well there is no condensation inside the engine or inside the exhaust or the fuel tank.