This was posted by Jonny (Mooney CEO) online, seems it’s back to business somehow on factory parts (mostly an issue for new models)
You can see the steel structure when you remove the interior. Removing the interior isn’t a big job, except for the headliner.
With a mirror you can confirm if there is suspected corrosion between the frame and the skins. I don’t want to even think of what removing the skins would involve.
Ours had some very minor surface corrosion on the frame, we cleaned it up, measured, painted the whole thing with primer, and replaced the insulation with modern stuff (the original junk is basically mineral wool, which is great for holding moisture once it enters through either the storm window or one of the vents). Think SB208.
We redid all the interior plastics with SEM paint, too, since they were already out.
The unplanned effects of deciding to add an audio panel ;-)
We’re almost done…
An A&P friend here is currently redoing the interior of his M20C and the steel tube structure looks to be visible with all the interior removed. Whether it’s enough to inspect or repair key areas, I don’t know. I’ll take a closer look next time.
Interesting to me is that prior to his disassembling the M20C interior, and despite rumors to the contrary, I found it more comfortable in the front seats than e.g. a Bonanza. Head room and control ergonomics are good for someone of my build. The back seats would be another story unless you are small.
The problem, apparently, is the cost of stripping off the skin to inspect the tubing. At the 10k mentioned, most people will avoid it for as long as possible. Can it not be inspected from inside, by removing the trim?
No different than any other steel tube fuselage, all of which also have the advantage of being easier to repair than any other kind of structure.
This issue is well known and has, in some cases, led to expensive repairs or even parting out of otherwise well kept airframes.
The “secret” obviously is to keep an eye on it. We check the steel frames in regular intervals and so far, in a close to 60 year old airframe, we have not found any damage or even start of corrosion. Hopefully things will stay this way. Corrosion is like cancer in a way, the earlier you catch it, the better the chance of stopping it in time.
That video briefly talks about some massive water ingress during flight in rain but I can’t see details of a resolution. Didn’t waste my life on every second of the video; they are done to make the guy money and are sponsored by the maintenance company he talks about. I had water ingress which turned out to be a crack in the front window seal (PR1422 or similar, ex-Socata) which I sealed off with Sikaflex 295. That stuff should work on a Mooney also.
A few years ago I saw some photos of a badly rusted (control linkages and everything else) 1970s M20 (they were emailed to me but were also publicly posted on a US site) and spoke to my A&P about it. He said this is the same on every 1970s Mooney he’s ever seen (in the UK). 50 years is a long time!
A lot of these issues are water collection due to blocked (or nonexistent!) drain holes.
Apparently not. He doesn‘t even mention the storm window, and he is a Mooney expert…
Anyway, it puts things like this (watch first minute or so) into perspective:
Kept the aircraft parked outside in always sunny 
Surrey for many months before addressing this. I know it‘s a different type, but still.
Mooney has one storm window, the seal on that storm window is bloody cheap and will leak, hence, needs regular changes, some aircraft still have original seal from 1977 (maybe they never fly in rain?), when the storm window leaks, it’s water end up over the cage steel or stagnate over the (wet) fuel tanks and may lead to corrosion on cage and leaks on (left) fuel tanks, the latter issue is very common on Mooneys and tend to require tanks reseal or baladers every 10-20 years…this can happen even if the aircraft is sitting in dry weather and hangar, a cheap fix of that storm window would have saved the show !
Everyone who flies Mooneys tend to be well versed on these two items: the smell of the fuel and cage corrosion, how to inspect & treat are well documented, I think way before YT videos were around
A pity people can’t see the large number of “toys flying out of the pram” posts by certain individual(s) 
Watched the video. Very interesting. M20, only pre-1976. But that is a very large number of aircraft. What happened on/after 1976? Did they put wax into the steel frame? 10k cost of the inspection!
