Knives should be kept sharp and cleaned periodically. Rust is not a patina. It's a cancer and should be removed completely and immediately. It only takes a couple of days at most to force an "age patina" on a knife. Without having the knife in front of me it's impossible to say for sure, but the black stains remaining on your knife appear to be just that; staining -- and neither rust nor patina. From a use standpoint, staining is neither here nor there. I'm not a collector and know very little about it, but my understanding is that a little bit of staining doesn't hurt the value of an old kitchen knife
You don't need to file the edge. Ordinary sharpening of the sort which reveals fresh steel will clean it completely (by definition). You'll probably have to go through at least a couple of use/sharpening cycles before the knife gets close to achieving it's true sharpness and durability characteristics.
The knife will wedge no matter what you do. Since that's pretty much [ahem] the point of a cleaver, there's no need to go acute. You can sharpen to a durable 25* - 30*. A double bevel is more durable, but if you're not going to use the knife every now and then it might not be worth the trouble. The best level of finish is fairly coarse; anywhere between a Norton Fine India (the equivalent of ~750 JIS) up to no more than 3K (JIS) is good. I follow my Fine India with a Soft Arkansas (~1K JIS) because the natural stone's edge seems to last longer and be more tolerant to steeling.
FWIW, the right way to use a heavy cleaver for making difficult cuts is to place it where you want to go and lean on it. Don't use it to vigorously "whack" at stuff or you'll bust your board and possibly your counters as well.
Ideally, you'll use the knife enough so that you have to steel and clean and oil it now and then, and that should keep the metal healthy. If you're not going to use the cleave enough to store it open, oil it, wrap it in newspaper, and try to remember to check on it at least every six months or so. Even if you don't use it you should sharpen and oil it (including the handle) at least once a year.
All of this reminds me that that I probably haven't used mine for a couple of years. Some example, eh?
Edited by boar_d_laze - 3/2/12 at 8:32am