I did wheel fender only plastic part because I have anticorrosion wax on car metal frame. Definitely recommend something with thickness up to 5mm- not much space.
THX for this very important information. Then there is no need for me to get Armacell at all. Will put AlyButyl on the (metal) inside the fender and will exchange the rear plastic "fender shell" with those made from felt.
Inside I will use acoustic felt then. Nothing else.
THX again!