Sharp is Good
The most important feature of the knife is that it be sharp; razor sharp is a good start. In a recent post on the Crispin Colloquy, D.W. Frommer II, a professional western bootmaker whose skill and philosophy I admire a great deal, said the ability to sharpen a knife is one of the most important skills a bootmaker can have. His reason was that without a sharp knife you can’t cut an accurate pattern, or make a clean skive and that your seams won’t go together as well because they lack clean edges. The ability also extends to awls and other edged or pointed tools the maker uses. Without being really well sharpened the tool may perform, but it won’t perform well and that will show in the final product.
A blunt knife will drag on the leather you’re trying to cut, which will distort your pattern and leave a messy edge. Don’t be fooled into thinking that sharp knives are more dangerous than blunt ones. Blunt knives are much, much more dangerous than sharp ones because you have to apply far more force than you do to a sharp one, and they won’t bite into material the same way, making it more likely that you will slip and cut yourself or at best slip and slice into the piece you’re cutting. The knife should be sharp enough that you don’t have to apply much pressure at all to get it to cut.
There is nothing to be gained from cutting things out in one pass. Take your time and make a number of controlled, accurate, shallow cuts rather than trying to cut through the entire thickness at once. This is particularly important for sole leather and patterning cardboard. A properly sharp knife will often cut through uppers leather in a single pass without requiring excessive force, but if it doesn’t don’t worry. Just go back and do another pass.
There is a lot of information out there on how to sharpen a knife, so I won’t go into it in great detail here. Have a look at these sites:
In woodworking circles there’s a scheme called “Scary Sharp” that involves using fine sandpaper glued to glass. It’s way cheaper than forking out for high quality sharpening stones and I think this could also be applied to knives quite easily.
I use a spyderco sharpening system that uses ceramic rods held at an angle in a plastic base:
It works pretty well, the fine stones put a good enough edge on a blade to go straight from them to a strop and produce something that is well sharp enough. The brass gaurds are there for a reason, if you forget them you’re liable to miss the ceramic rods and end up with neat scars like this:
Joking aside, I was damn lucky not to sever tendons when I did that. It took 5 stitches to close and took my hand out of commission for a good couple of weeks. I did it because I was in a hurry. Take it easy and you’ll be a lot less likely to hurt yourself.
When a knife you’re working with starts to get a bit dull it doesn’t usually need to be fully resharpened. A touch-up on a strop will bring it back to sharp pretty fast. A strop is just a piece of leather (use smooth stuff not something with an embossed grain pattern) glued to a bit of wood with some cutting compound rubbed into it. They polish the edge of a blade so that it’s smooth and the smoother an edge is the cleaner and easier it’ll cut. I strop craft knives before I use them otherwise I find they aren’t quite sharp enough. Strops are also critical for keeping a good point on an awl. The one I use for my awls looks like this:
The grooves in it are from dragging the awl points across it. One thing about using a strop, you drag backwards trailing the edge, unlike sharpening where you push the edge forwards across the abrasive. You can get the cutting compound from a hardware store, just get the low speed stuff and get the smallest stick you can — you only need a tiny amount. The other, probably better, thing you can use is jewellers rouge if you have a jewellers supply in your town.  Lee Valley Tools sells a compound specifically for using on strops aimed at woodworkers. The needs of leatherworking blades and woodworking blades are pretty similar so this will be a safe bet. They also have a wide array of sharpening equipment.
Knives I Use
I use a few different knives depending on what I’m doing, none of them are period — something I will eventually fix — and at least one of them really needs to be thrown out to force me to learn to use something better.
The knife I reach for more often than I should is a small snap-off blade craft knife
These have the advantage of being cheap and sharp enough out of the box that they only need a little stropping to get really usefully sharp. They do, however, have one major disadvantage for doing any precise work — the blade flops around in the handle like a fresh fish in the bottom of a boat. This makes it impossible to cut square edges, even if you have really steady hands. This causes problems when it comes to cutting edges for butted seams, and when cutting thick leather. If you cut on an angle it’ll be bigger on on side than on the other and the edges won’t meet tidily.
A better option for something reasonably sharp out of the box are the little craft knives with a screw clamp mechanism to hold small blades in the end of a handle. I think Americans call them Xacto knives.
I don’t have one of those though, instead I have a couple of Swedish scalpels I got at an army surplus store. They get scary sharp but are a little bit chunky at the spine of the blade. Craft knives have very thin blades which can be useful.
The other knife I use a lot is a so-called “shoe knife”. I got my first one of these for a fiver at a hardware store and was given a box of a dozen by a friend who got them on trademe. They get plenty sharp but need frequent touch-ups during use to stay that way. I would advise people starting out to get one of these and learn how to sharpen it.
It’s not stainless so there’s some kind of staining on the blade from cutting damp veg tan. I’m probably going to cut the blade of this one back to about half it’s length to make it a bit easier to control.
A kind of knive you’ll see mentioned a lot is the round or head knife. This is a direct descendent of the trenket used by medieval shoemakers, which had a different handle configuration than the modern ones but was the same basic idea. I have a cheap crappy one. It sucks. If you’re going to fork out for one of these get a half decent one like an osborne or something.
I have never really gotten my head around how to use this thing properly. The major reason being that I’ve never managed to get it sharp enough. I don’t have a mechanical wheel sharpening system and I think you need one to get a good edge on one of these. There are instructions on how to cut with one of these in one of Al Stohlman’s books but I haven’t seen them.