After playing Dr. Science for a bit I decided that the best way to move forward with the SKS gunstock was to start the actual application of the finish. As I want to leave my options open I decided on a basic Tung oil sanding coat and then a next sealer coat of Tung oil for the main part of the stock. The handguard needed a bit more than that, however, as the wood it is composed of is very much lighter than that of the rest of the stock.
The procedure I used for that was to apply a basic Tung oil application (at 1:1) with solvent go through a 45 minute set and then wipe process, then another wipe down 4 hours later and then let sit overnight. This is the wipe/wipe/cure process which will be used throughout for all oil applications. I also did the entire gunstock with that (external and inlet areas), then sanded them down the next day with 400 grit sandpaper. Unsatisfied with the tonal qualities of Tung oil on the handguard I then applied a Walnut oil finish to the handguard, compared it to the sanded stock and found it to still be much lighter than I wanted.
At that point I decided to fall back on my favorite of shellac and put on a 50/50 mix of 1lb cut orange/garnet shellac in two coats to the handguard and sanded between coats, let those dry to a very light amount of tackiness and then sanded down the second coat. That did darken things up a bit and yesterday was the application of a second Tung oil over the handguard and gunstock, and did a brushing in of one loaded acid brush with Walnut oil on the left/rear portion of the butt part of the stock in the arsenal numbers area. That went through the standard procedure and received a 0000 steel wool replacement work over along the grain and then some more 400 grit sanding in selected areas to lift some of the darkening that happens with oil layers.
This is the result:
This is the side that received only two coats of Tung oil for the stock, the handguard is as described.
On the shooter's side of the stock the rear of the gunstock is still lighter, but with somewhat more pronounced brown tones than it had previously.
The underside of the stock and I've turned the handgaurd around, as well. There are some lightly rough spots at the left edge of the inletting near the rear screw hole that I have left rough with but minor sanding. I didn't feel it needed any putty treatment and shows wear on the wood, as does the butt end of the stock with minor chips missing from prior use in its former life.
From the top of the stock one can see the tonal gradations front to rear a bit better on the non-shooter side of the stock.
The rubdown with steel wool replacement and light sanding to lift tonal qualities in dark areas of the stock also revealed a somewhat slick feel to the stock, meaning the oil needs a bit more curing time. I will check it again in a couple of hours and then decide on what the final application(s) should be.
Putting on my Dr. Science hat for a moment:
I am pleased with a separate test of 1:1:1 BLO:Walnut:Cedar as leaving a matte finish, toning down the yellows but increasing the contrast with the browns vs inter-grain areas of white wood. Aromatic oils do help to thin down the other oils I used and decrease drying time and may actually speed the curing time by being aromatic. Because of the aromatic nature, however, and the need for molecules to detach over time, it would not be safe to put on a finishing layer that did not allow such molecules to go through the coat. Thus I wouldn't trust a spar varnish unless it had some proportion (at least 25%) of a Cedar oil in it if I applied the BLO/Walnut/Cedar mix to it.
Linspeed has fallen out due to the fact that it was nearly solid in the glass jar, which indicates it had sat in some exposure to air for a time and cold not be depended upon for a final finish. It had progressed beyond oil to gel and solid, and there is no way to really reverse the chemical reaction once it has taken place.
I do have the Cabot Spar Varnish available and as it is a finish that flexes over time, may prove more suitable to a gunstock's final finish coat. As I don't want to run a multi-month test to see how that reacts with Cedar oil or with Cedar oil mixed into it, I hold that in reserve as a possibility.
Note that if the Cedar oil does actually decrease cure time, then it will also help to cure previous applications of Tung oil via that process. On my test board I used no solvent and the oils ran thin and fast at the 1:1:1 ratio, just as had happened in prior tests. Adding 1 or 2 parts of Citrus Solvent would thin that even more, meaning less oil and better if thinner coverage. For brushing purposes 1:1:1 is perfectly acceptable because of the low viscosity of both Walnut and Cedar oils.
A final way I might be able to darken the handguard is to add a tiny amount of lamp black (common soot) to the oil mix, which is a method a few violin makers have used to change the refractive index of their varnishes and help to absorb some light in some directions (due to the nature of soot it will not form a single angle alignment in a varnish or oil application) while reflecting it in others.
I have used shellac intermixed for non firearms purposes (like putting it on my router table) and have noted no problems with it. As it forms an impermeable layer to the movement of water and other small molecules (say components of oil breaking off from the parent structure during curing) and has heat sensitivity problems, I used it sparingly on the handguard. With that said Mosin-Nagant rifles I own have gotten far, far, far too hot to touch on the surface of their handguard pieces (I have gotten the blisters to prove that) and I know that cured shellac on wood surface is pretty tough stuff and not a wilting flower in the wood finishing department for firearms. It has not clouded or otherwise changed its clarity due to that amount of heat. Intercoated between oil finish may be a different story and re-doing the handguard would be very much faster than doing the entire stock, that's for sure.
Shellac is very much not what is used on the SKS gunstock, but worth consideration if it can perform the necessary functions of protecting the stock as well as an oil based system.
One long, long, long term worry is that oils do react slowly with the underlying wood and while cases of wood softening are rare, they do happen. If I had to redo the stock I would actually aim for a shellac sanding coat of platina or platina/light button lac or blonde shellac as a base for the stock and garnet for the handguard. After sanding could come the oil finishing which would allow the stock to have much of its wood protected by the shellac and perhaps allow a means for some lighter molecules to finish their reaction with their surroundings and not continue them. With that said there are some really beautiful oil finished rifles from more than 130 years ago that show no wood degradation, so much depends upon the wood, the oil, the time, the environment in which the stock has been stored/used.
I am still, at this late date, undecided on the final top coat composition.
BLO is traditional and only changes color composition slowly over time. It is the oil used on SKS gunstocks when the arsenal remembers to finish them, that is... or allow the BLO to actually cure before placing them in cosmoline...
Tung oil has performed very well due to its being a bit lighter in viscosity than BLO and only does a minor hue shift to the yellow as compared to BLO. It is not a traditional SKS oil, but looks good on beech wood more than 40 years old.
A mixture with Cedar oil promises some interesting components and may safeguard the stock against some longer-term biological attacks due to the nature of the oil.
Spar Varnishes are great for outdoor use and flex well with changes in climate, humidity and temperature so that the finish doesn't offer cracks to expose underlying work to the elements.
At this point it is a toss-up, but I can say that the stock does not look like Bubba had gotten to it. For that I would look at some of those lovely chrome paints after basecoating with white, and then putting on a thick automotive clearcoat.... hmmmm.... shiny!