LOL I know when I used it on my street/drag car it caused my headers to rust from the inside out because it held moisture inside the tubes. Jim
How could wrap make steel pipes rust from the inside out?
The steel is not porous.
Inside-out exhaust pipe corrosion is caused by frequent start-stop cycles where the pipes do not get hot enough to dissipate the corrosive acids that are present at a cold start. The pipes just get warm, then you shut it down, then the insides of the pipes build condensation, and that is when the corrosion begins. Running the motor long enough to get the pipes hot - and the motor up to operating temperature - is the'hot' trick. That's why an auto museum curator never lets the engines be started up unless they are going to run long enough to completely warm up.
Header wrap can, if installed improperly, accelerate rust-through from the OUTSIDE. Even though header wrap does not really absorb water like cloth does, it can pick up some.
Proper installation is to soak the wrap in hot water for as long as you can. Then you pull and stretch it while wrapping it on. Sometimes, it is easier with two people. You start from the top and wrap towards the ends so that the open edge always faces to the rear. You secure the last wrap with stainless safety wire - two or three wraps, and twist - the wire kind of disappears into the wrap and it looks better than a crude hose clamp.
Then you run the engine and let all that moisture in the wrap boil off. The wrap will shrink slightly during this process. When you are sure all the moisture is gone (no more steam), let it cool and spray it with a few coats of header paint. It soaks in, seals all the fuzz, and acts as a moisture barrier. Run the motor again to cure the header paint. Over time, you can renew the header paint as needed.
This is the way we do it on custom bikes. It will work on an MV, too.