First, soak the latches in diluted muriatic acid until all the coating, scale, etc. is gone. Then rinse them off, and start misting them every time they dry out. After about a dozen times, they'll get pretty rusty. Once they've rusted up good, make a mix of 1/2 tung oil, 1/2 boiled linseed oil. "That's the original recipe for Rustoleum," the fellow informed me. Rub that on and bingo, you've got 100-year old latches.
Here is the patina factory, on a board over the wood stove:
During the hours of mistification, I oiled and assembled the lids–oak rim, cedar planks. Then I trimmed the canvas inside the hatches, folded it back on itself, and tacked it to the rims:
After about twenty or so mistings, the latches did just what the guy said they would, and the tung/linseed mix worked beautifully on them:
It was only as I was finishing up that I figured out I had left one latch in the vise where I trimmed it yesterday. Poor thing was an hour's acid bath and twenty mistings behind it's friends. I guess I can put that one on tomorrow.