A guy that posts on THR bought a couple of Civil war period Colts percussion pistols that had been hidden under the floor of a tack room in a barn. They were wrapped in rags, and were loaded. One of the guns had some rust on one side of the cylinder, but both were in good condtion. He replaced the caps and shot them. The one with the rust on the cylinder had some hang-fires, but went off, the other one fired all six fine.
Another guy on Leverguns did an experiment with some primers and percussion caps. He put 10 each of 2 different brands of caps and centerfire primers in a cup of water and left them for a week. The then took them out, shook them off, blew them off with his breath, and they all fired. The caps on an otherwise unloaded revolver, the primers in empty cases. I think oil is more damaging to powder or caps than moisture.