With the heat treating done, your blade is nearly complete and ready for "handling". However, it's pretty ugly and not exactly finished looking. If you're a perfectionist, you'll do well here. If not, you've gotta work on being one. More on that later.
The last grit you left off with before heat treating the blade was the lowest (grittiest) possible, probably 80 grit. If you have a machine grinder, go up to the next level, 120 or so. Grind the whole blade, and go slow. There are three goals you have at this point:
- Even out anything you missed last time you ground the blade
- Eliminate any trace of oxides (anything not steel silver)
- Thin the edge, as thin as you can safely get without ruining the temper (overheating)
This time, hold the blade with your bare hands, dunking in a bucket of water whenever it gets too hot to hold. Any heat over the temperature you last tempered at will soften it more than you need, making the affected area dull right away in use. It's not too hot until you see the steel change color (a big danger especially with high speed grinders), but by the time that happens it's too late. Be safe and keep it at room temperature, dunking in water periodically. If you're using an angle grinder, put the grinder itself in a vise, and grind the blade by hand from there. With an angle grinder, make sure the edge is pointed away from the direction of spinning, not into it, so the edge doesn't get caught by the grit and flung across the workshop.
Keep switching out grits, going periodically higher, for as high as you have grits for. Remember, the finer the grit, the higher the number. Also with the higher grit, comes more heat generated friction. Cover every bit of the blade. Make sure the plunge line (part where the bevel and cutting edge ends and transfers into the ricasso) is even on both sides, and match the blade perfectly to your design.
Now make sure whenever you switch grits that you eliminate the scratches from the previous one. I'll explain that in the context of hand sanding, so read on.
Most bladesmiths agree that this is the most tedious part of bladesmithing. But also, it is the feature that really makes the blade look professional. It's a pretty simple process, it just takes a while. Keep going, and don't ever settle for good enough. You're done when you can't do any better.
Clamp the blade down on its side. Spray with WD-40 or Windex (some people use water, but these are more effective), just enough to wet the blade. The liquid acts as a cleanser; when you start sanding, particles of worn sandpaper and steel get clogged between the grits of the sandpaper. From time to time while sanding, wipe off the blade and sandpaper, and spray down again. Old rags come in very handy in the workshop.
Use block of wood with a flat face, and wrap the sandpaper around it (start with a grit one level coarser than you left off the grinder with; if you stopped the grinder at 220, use 120 or 180 grit sandpaper). Start sanding, putting pressure on the block, from tip to plunge and back. For me, the first grit takes the longest, but is easiest to tell if you're done, due to the old and new grind lines being perpendicular to each other.
Keep sanding until there is no trace of the grinder marks. Get them out now; it's impossible to eliminate them later. This is part of the finishing stage of the blade, and apart from the shape and design, is the one thing that will make the blade look the best it can be. Perfect the hand sanding, and your blades will look exceptional. Perfect is good enough.
Moving up the grits
With the grinder marks eliminated, you're going on to sanding, like with grinding, up the different grits, to no less than 400 grit. There is a large factor to keep in mind here though; if you start at 180 grit, and move to 220, you must make sure that by the time you're done with 220, all the 180 scratches are gone. This takes some visual explanation.
Imagine zooming in, to where the line of sight is on the surface of the blade, looking down it from tip to plunge, microscopically. Now remember, sandpaper works by little bits of grit (sand particles) scratching off bits of steel, leaving scratch marks behind. The bigger the grit, the deeper these scratches. Having just finished sanding at 180 grit, the surface will look like this, from the aforementioned point of view:
It's an exaggerated diagram, but you get the point. The valleys in the steel are where the 180 grit scratched in; the mountain tips are the spaces in between grits. Your goal in hand sanding, is to make the surface completely flat, or close to it.
A common mistake people do is then sand at 220, then move on to 320 before they're done with the 220. This is what unfinished 220 looks like close up:
It's the same mountains and valleys left by the 120, just in this case, the mountain tops have their own smaller mountains and valleys, left by the smaller 220 grit. The mistake, is to continue on to the next grit; all the while, the scratches left by the 120 grit remain. The correct sanding is to keep going until you've reached the depth of the 180 scratches:
As you can see, you keep sanding, and you remove the 180 mountains completely, replacing them with smaller 220 mountains.
And so, the cycle continues, on to 320, then 400, and so on. Eventually, around 1500 or so, the mountains are so small, then are easily removed by leather and a buffing compound; a sort of paste with microscopically small grits. This leaves you with a polished (completely flat) surface. The reason it gives you a mirror image is because light is reflect off the surface, rather than uneven mountain sides.
You don't need to go all the way to polishing; if you ask me, a blade often looks better with a "satin" finish; very small mountains and valleys (scratches) that are nearly flat, but not quite. It gives the blade character and looks smooth and flowing. This means usually stopping at 400-600 grit. Play around. Go to what looks best.
But how do you know when you've finished a grit? The light bouncing off the sides of the mountains look just the same bouncing off a deep valley as a slightly shallower one.
So, a trick I learned is to change direction of sanding every time you change grits. If the first grit is straight from tip to plunge, then sand the next one slightly askew. So if 180 scratches are in A direction, and you start sanding with 220 at B direction, you know you can move on to 320 when the only scratches you see are in B direction. It's pretty simple, but it works.
With the sanding done, the only work that needs to be done on the blade itself is sharpening, which is the very last step in knifemaking. Next up, will be fitting a guard or bolster.