[DaninVan]

Hey, Dave; welcome!
On the "little swinging doors" thing did you mean the roof rafters?
Is the ceiling in the garage unfinished? Not clear on why you'd want doors at the tail ends of the rafters???
Normally there'd be solid blocking between the rafters over the top plates, or some means of allowing venting into the attic space year round. The blocking or vent screening prevents critters or insects from getting in.
It's all just theory until a squirrel gets in and sets up housekeeping...or wasps.

 

[DaninVan]

It occurred to me that maybe you meant the gable end spaces? To allow for passing lumber through?

 

[DaninVan]

Dave; you're 'preaching to the choir'; I live in the Pacific Northwest, so does Herb Stoops...we invented rust...
Insulating your shop will help dramatically in controlling the humidity as it's relative to the air temperature. Keep the air inside moving with a small fan. Don't cover your cast iron tables up with plastic, it seems to make the problem worse.
Try using a product like Boeshield as Tom mentioned, or even furniture paste wax (no silicone content!!!).
If you decide to insulate the roof rafters, not the ceiling joists, please have a chat with us first. We'll save you some potential misery.