Metal Finishing: Polishing with Abrasive Stones

Over the years, I’ve attended a number of the NRA’s one-week gunsmithing classes, both at Pine Technical College in Minnesota and the gunsmithing school in Trinidad, Colorado. My interest has been in work related to rifles exclusively, and these classes have been rewarding and useful to me as a hobby gunsmith. One of the classes I took was taught by a member of the Guild of Custom Gunsmiths, whose work was restricted to metalsmithing.

Besides guiding us through the making of jigs and fixtures for detailing metalwork, we spent significant time learning to do metal finishing. This is where we learned to appreciate and use stones to preserve corners and edges while bringing the surfaces to the required polish for bluing or color case hardening. The stones we used are widely used in the tool-and-die industry for polishing injection molds for plastic parts, among other things.

As beginners, the instructor furnished us with stones that were 1/4inch square by 6 inches long and made of soft silicon carbide in grits from 100 up to 320. The stoning was done with a thin oil, which quickly worked up into a slurry that must be wiped off periodically in order to see our progress. It didn’t take us very long to appreciate how efficient and effective these stone are in metal finishing.

I’ve always considered polishing metal by hand to be an onerous chore’ve ry time consuming and labor intensive’especially if you want to get the results seen on the best custom rifles. I’ve found that, due to fatigue, I’m not able to do this for long periods of time; an hour or two at a time seems to be my limit. The main reason for this is, in order to exert the pressure needed to make the stone cut, they’re held in the hand and fingers much like you would hold a pen or pencil.

Later, I ordered a stone holder, and found that this accessory makes using the stones a lot easier on the fingers. By using the holder, you will also get more use out of the stone before it has worn down too much to be useful. When I got home from the class, I sent fm Congress Tools the company from for the class were obtained. I ordered a goodsized assortment of stones that will last mealong time, and didn’t even cost as much as a 100sheet pack of abrasive cloth or paper.

Congress offers a huge variety of stones’ silicon carbide, aluminum oxide, India, ruby, ceramic, and so on’all in a great range of grits, sizes, and shapes. I stuck with what the instructor recommended, and bought mostly soft and medium-hard silicon carbide, 1/4 by 6 inches long, in grits from 100 up to 400. I also purchased a few round stones and a few in hard aluminum oxide and India just to try. These have all worked well for me for many years now, and I’ve found no need to change.

As an aside, though, some years ago I took a short class in single-shot riflesmithing from a highly regarded gunsmith who mentioned that he did not use stones. Instead he uses strips of wood about 15 inches long or so, indifferent shapes and sizes, with abrasive cloth glued to them. I tried this and they do indeed work very well, The handle, also available from Congress, makes the stones considerably easier to handle.

By cross polishing when changing to the next finer grit, you will be able to see the scratches and stoning marks to remove them, but on some convex surfaces this will be difficult to accomplish. The soft silicon-carbide stones quickly wear to fit the contour you are stoning and the adjustment attained makes it easier to stone convex or concave surfaces. This is a nice feature.

When the surface is stoned to the degree of polish desired, a few minutes with 320-grit abrasive cloth or paper will blend the stoning, and the metal should then be ready for bluing or color case hardening. The I but I found these strips to be time consuming to make up and the abrasive cloth wears out quickly, whereas the stones last for along time. It’s imperative, of course, to keep the work flooded with a thin cutting oil or else the stones will quickly load up and become unusable.

I’ve found that kerosene mixed with a little brown cutting oil works very well. l apply the oil to the metal and start the stoning, constantly wiping the metal off when a thick slurry starts to accumulate so that I can see my progress. I don’t dip the stone in the oil, because the abrasive dust will quickly contaminate the oil. Instead, I use an eye dropper to drip the oil onto the metal. instructor in the NRA course told us that, if we planned to cold-rust blue, there was no need to polish finer than 320 grit.

For my purposes, I’ve found this to be true and have done both cold-rust and hot-dip bluing and can see little or no difference when the metal is polished to 320 grit. Over the years, I have accumulated some nice buffing equipment which is very effective, but I no longer use it very often except on non-gunrelated projects. Once you’ve seen the results from hand polishing, you’ll be spoiled, and you’ll come to appreciate the fine finishes found only on the highest quality custom guns.