corosion during storage

How about, instead of trying to seal it, be proactive and apply corrosion-inhibiting materials? Grease or oil of some sort. The interiors of aluminum airplanes are sometimes sprayed with such.

And why would the PIK20 be more susceptible than other types? If the geometry inside of it causes condensation to specifically drip onto some fittings, perhaps can add barriers to that dripping. E.g., a bead of caulking a few inches uphill from that spot may cause the dripping to happen where it is harmless.