Waxoyl is a cavity wax , and is very good at that job.
It also has to be applied with a suitable air fed gun /applicator not the issp poor squirty thing that is often suggested . I use a trigger oil can with tube extension too.
Applied properly and regularily it WILL prolong the life of any car .
I sold an Opel Manta that I owned for 35 years in 2012 and it is now owned by a chap 35 miles from me . It has never been welded and is as good as new due to waxoyl. I have a Transit van that I have waxoyled since I got it and nearly all of a similar age are gone mine flies through the stringent commercial annual test each year .
My Omega has been waxoyled everywhere doors/sills/wheel arches/tailgate/under liners/chassis legs and is unmarked .I smirk when I read about rust in the arches and sills because that is down to no measures being taken to prevent it .
My BMW is flooded with the stuff .
I listen to all those who decry Waxoyl and just laugh ....
Waxoyling any vehicle is a really messy dirty and time consuming task and if you don't get filthy when doing it , you haven't done it right .
You get nothing for nothing when it comes to preventing rust on a vehicle .
Oh , and if you are too tight to buy it make your own .
Grated candle wax , chain bar oil and white spirit mixed together and kitchen blender . The consistency can be adjusted to suit any application .Google it because it's good and it works , I been using home made waxoyl for several years now.
Don't use it on any surface that will be abraded by road spray , clean /prep/dry and apply several layers of Gravitex for those areas .
And don't think it's a one hit solution , you have to keep on top of it by regular inspection and reapplication if necessary .
There .....!