The humidity in the air eventually enters the engine of a parked airplane and does it no good. It is probably even more prevalent on airplanes tied down outside of nice dry hangars, because the wind constantly bathes the airplane/engine with nice fresh, humidity-laden air. Even in desert regions, moisture exists and does it's bad thing during the cool-down periods of the evening. That humidity eventually finds it's way up your exhast and onto your valve stems and into your upper cylinders where those nice shiney steel walls and valve stems develop a nice light red/brown coating of Fe2O3.
Add to that, the wasps and mud daubers that see airplanes as wonderful places to incubate their young larvae. Those nests have caused many a airplane to have troubles. Only a year or so ago, an IA acquaintance of mine had the pleasure of a visit from the fuzz who investigated him and his mx records for a week because a 172 which he'd just annualed crashed on takeoff due to a nest inside the carb air box that sucked into the carb and shut it down about 50 feet into the air. It turned out that the nest had been constructed in less than 2 days of sitting outside his shop for the owner to come pick it up. (The proof was the receipt for the new airbox, for it and the carb had just been replaced during that very annual.)
Your carb air box has a small approx. 2" opening to atmosphere on it's lower surface. This allows the air coming to the carb from the heater muffs to exit downstream when carb heat is not selected. It also calls to wasps and mud daubers, to "Come! Free real estate. Good flying schools. No taxes.
Anyway, there are many more areas for the buggers as well as places for humidity to get inside your engine and cause rust, etc. How do you know, for instance, there are no mud nests inside your exhaust muffler blocking exhaust gases? That mud bakes into a nice, hard brick and can remain firmly attached to the inside muffler baffles. (And you were worried about which way your fangs were pointed?)
Here's a simple, cheap method I use to cut down on such things. You know those cheap, plastic "scrubbies" sold in the kitchen-supply section of the grocery store? (One brand-name is "Tuffy". but even cheaper copy-cats in wonderful, bright colors are also available.)
I use them to stuff into my wing's leading-edge openings to keep nests out of my cabin vents. (Got a nasty surprise one time after take off. THAT was a lively time!)
I also use the larger versions to stuff into my cabin heater and cabin air intakes inside my cowl. On those scrubbies that are installed deep into the cowl on such openings, I tie a long sturdy nylon string thru/to them to pull them out during pre-flight. The opposite end of the string has a bright remove-before-flight streamer dangling from it, but cheap, common surveyor's tape will also work just fine so they can be easily seen/grabbed/pulled for preflight.
For my exhaust tailpipes, I simply stuff a wax-paper "Dixie" cup into them upside down. This keeps insects and humidity out of my exhaust and cylinders. (On my small single-cylinder engine stuff like lawn mowers, tillers, generators, etc., after shut down I gentily pull the starter cord until I feel the cylinder on top-dead-center to ensure that both valves are closed. On the airplane, of course, the throttle is closed to limit ingress of humid air.)
The bright scrubbies are easily seen during preflight, and the Dixie cups are long enough to see and would just blow out anyway, if forgotten.
As for all the other openings in the airplane such as controls, flaps, etc. where those buggers like to go....I've been experimenting with "tobacco bags" filled with moth balls. (Tobacco bags are small cloth bags with a string-closure which can be purchased at aircraft parts-supply houses. They are commonly used by shops to hold all the small screws, parts, etc. during inspections because they can be hung from the open inspection holes/covers etc.) I hang the bags all around those obvious areas such as aileron/flap ends, tail cones, elevator/rudder balance "horns" etc.
I can't quite swear it's 100% effective yet, but it seems to help. (I have a "control" aircraft* in my hangar with no protection at all, and I'll let you know the results of the experiment after this season's wasps have moved on in the winter.)
Just thought I'd pass along the ideas.
*Ol' Gar has parked his C150 at my place, and just to give him something to worry about I re-named and call his airplane "Dauber Decoy".
