I spent more than 40 hours removing the entire interior of a 177, then blending out many corrosion spots and pits. I had to draw grids of 1/4" squares on all affected areas of that spar carrythrough. Then an NDI tech came with his ultrasonic thickness measuring equipment and read out the thickness in every square, hundreds of them, which I recorded on photographs of these areas. Those were sent to Cessna, whose engineers evaluated it and passed it. Then I had to use the primer specified by Cessna on the carrythrough; that alone takes a lot of time to mask off the whole interior to prevent overspray onto the windows and instruments before spraying. Then reassemble everything.
Well over $40K for all that, on a $70K? airplane. There are no new carrythroughs available.
Beware.