Don't buff a hamon. It can mess it up. Sand by hand after 400 grit to around 1000 and etch. Work the areas that you want shinier individually up to 2000 grit. There are all sorts of oxides that can be used with a small cotton pad (I use makeup remover pads) to enhance the color and look of various areas.
Here is some old posts I made on creating and polishing a hamon:
Yaki-ire:
1:4 ferric/water is too strong for working on a hamon. I use 1:15. Other acids are lemon juice, vinegar, or 1:100 HCl.
Brine is water and salt. To make an 8% solution, dissolve 350 grams salt per 5 liters water. Warm the brine to 40-50°C.
yaki-ire is what a hamon quench is called in Japanese. It is a very fast and violent quench, and you will crack and break blades a lot while learning it. Most folks learn to do a regular quench for a long time before attempting a hamon quench, so with only a month of experience, you can expect a lot of failures in yaki-ire. We often call the sound of a failed quench, "The dreaded PING'". It makes a sound you can hear and feel in the tongs - a sharp PING or TINK.
Heat control in a quench is hard to teach over the internet. If you have an experiences maker to teach you it will help a lot. A forge will overheat a blade really fast. Using a magnet to test when the blade is ready to quench is a good way to know an approximate temperature. When the blade stops sticking to the magnet, stick back in the forge for a few seconds and then quench. Obviously, a HT oven is far better, but most new smiths don't have one yet.
Dull spots that don't etch:
There are lots of reasons the metal will not etch dark. Decarb is the most common. Basically, the higher the carbon, the darker the etch. Structure also affects the degree of darkening. Martensite etches darker than pearlite. In a hamon blade, the edge is martensite, and the upper part is pearlite. The place where they meet and mix is the hamon. The frosty look and little islands of bright spots of the hamon and area around it are from fine and coarse martensite crystals of nioi and nie.
A polish techniques that can develop a hamon:
Sand by hand evenly and slowly. Using water stones is a great system, but requires a lot of skill. Good quality sandpaper, a bowl of clean water with a few drops of dish soap and a 1/2 Tsp of washing soda ( to retard rusting), a clean and comfortable place to sit, backing sticks/blocks to hold the sandpaper on, and lots of elbow grease are the main ingredients.
Sand evenly and in the direction of the hamon (lengthwise, not crosswise) and go up the grits to 600-800. Give the blade a quick etch in 15:1 FC or full strength white vinegar. This will help you see where the features are. You should see the main line, and hopefully any secondary lines and some wisps ( if they are there).
Continue sanding the blade at 1000 grit, rubbing a bit harder above and below the hamon line than directly on it. At 1000grit ,the features should start to show. Rub the hamon line with a weak etchant. Use a bit of thumb pressure and go in tiny circles. Look for lines and wisps that are showing now that you did not see before.
Continue polishing to 2000 grit. Depending on the surface desired you can quit here or go up finer. Etch the line to see what has shown up at the end of each grit . You may think you are removing the hamon as you polish, because the surface just gets shinier, and thus the hamon disappears. The etch will show what is there, and the details will be enhanced in the next step.
Now that the blade surface is prepared, it is time to "develop" the hamon. All the features that are present are locked in the steel, and are waiting to be made more visible. This will be done by oxidizing, and changing the surface of the steel in different ways. A word about the work area...CLEAN. Let me give you that word again - CLEAN.
Any grit sitting around from previous sanding will come out and haunt you. Clean everything up well, wash all tools and blocks, wipe off the table and chair arms. and make sure the blade is C_L_E_A_N.
On a very simple hamon ,with little or no detail beyond the line, you might just carefully work the surface between the edge and the hamon to make it frostier than the shiny bevel above. This is not the same thing as the fake hamon on many cheap blades that is sand blasted on.
You will carefully work an abrasive compound (the paste from a tiny flat stone in shiagi togi), to dull the shine on the surface. On a hybrid polish, use a fingertip to rub a fine grit abrasive that will leave the surface less reflective. This can be anything from finely ground scale collected of the anvil in forging, Nagura stone "mud" paste, fine valve grinding compound, or 4-6 micron diamond paste. The fine grit in the compound will leave a smooth but slightly frost surface. Carefully follow the hamon and do not go above it. This looks good and sometimes is all you can do with a very simple hamon. Some folks quite here.
If you don't like the completely "white" surface to the edge, use some Flitz or rouge and careful polish below the hamon. Wash everything and give the blade another etch. Now use just a tad of Flitz, and re-polish the blade, staying off the hamon as much as possible.
For a hamon with lots of activity, the process goes the same up to when you switched to the abrasive compound. Instead of making a frosted surface, you will try and coax the hamon out by polishing the three surfaces with different materials. The surfaces are:
The area above the hamon
The hamon
The surface from the edge to the hamon
Each surface should be polished in a way to enhance the hamon. The way to decide is far beyond this (already long) post. Try several polishes and etchants, and observe what the steel looks like after the polish. red oxide leaves a darker look, green brighter,acid etchants white, FC dark. At each step, you often have to re-etch and then rub off the oxides with a fine compound, like Flitz. One standard (well, there are no standards) look is a shiny edge, a white hamon, and a darker bevel.....or the reverse of that.
You may need to employ strategies to keep from polishing areas that you don't want to change. Applying white nail polish over the hamon area after you get it just the perfect frosty look, can allow you to polish and etch above and below without worry of ruining the last 4 hours of work. Remove the nail polish with acetone when you are done. The same method can be used to protect the shiny and darkened bevel from damage while re-working the hamon and lower surface.
A piece of cardboard or plastic held by your right hand at the boundary of the hamon will keep the compound you are using with your left hand (to make the hamon white ) from dulling the brightly polished bevel. After the hamon is worked, the same procedure (new piece of cardboard/plastic) can protect the hamon while you polish the edge.
Enhancing the hamon :
There are many ways to enhance the hamon. One is to use dilute nitric acid on the hamon. You can mask above and below the hamon to etch only the desired parts. This is basically how the faux hamon on some cheap Chinese katana is done.
The use of an oxide powder to enhance the look and color of the ji and ha is the common method. I have covered much of that in past threads that can be found by using the custom search engine. There is also info in the stickys on this.. A good kit includes rouge (red oxide), chromium oxide (green oxide), magnetite powder (black oxide), cerium oxide (pink oxide), tin oxide ( white rouge), fine alumina powders ( alumina A and B), and various fine silicon carbide abrasives ( valve polishing compounds) are the most common. Choji oil (clove oil) or plain mineral oil is a good carrier. The whole kit can be put together for around $50-100 and will last for many. many swords/knives.
EDM and die polishing sticks can be found in almost any grit and size. These can work like hazuya and jizuya stones. Cratex sticks can be used, too.
