Until just a week or so ago I was custodian of a metalized Dash Two. Both the wings and the fuselage were metalized with the Ruleto metalizing kits. Assuming a similar kit, the fuselage steel structure should be in good condition – the metalizing kit stands the skin away from the metal tube structure to create a shape the sheet metal can be draped around so there is noplace where water can be held against the steel tube structure. Any water that does get inside the fuselage runs down the belly towards the back and drips out the open area that the tail spring emerges from – the water never touches the steel tube structure. You can inspect the tail post area fairly well from the removeable triangular panels below the horizontal stab on each side, plus up from the tail spring area. You can also look down the tailcone from the cargo compartment area assuming the plane in question has a cargo compartment. As already mentioned, it is not a big deal to drill out some rivits and temporarily remove some metal skin if you feel the need to inspect more closely.
For the wing, the sheet metal is rivited directly to the wing ribs so inspection is more difficult. As already mentioned one can remove the fuel tanks and/or create new inspection holes for access, plus a boroscope can tell you a lot by carefully fishjng it around from whatever access points you find or make.
Bob and Pat and Blue Belle the fabric 108-3